Balketh - Allow me to absorb this coffee as I read your post. I'll edit this in a second.
Also: on perusing your spreadsheet, would anyone care to explain the purpose of Soothe/Bask/Enkindle? I don't understand a situation where I'd want to lose exposure for such a relatively long period of time in-gmae, only to regain such a small amount thereafter? What's the thematics behind this? The names suggest they're to do with warming up, and yet you spend the majority of the spell colder than you were.
They are the magical equivalent of alcohol. You lose (reduce, "restore") X exposure, immediately, which is a state that lasts Y seconds. After Y seconds is up, you gain (increase, "damage") it all back, plus 5. It was a trick of the mind, and the sudden change shocks you. It's a net loss situation. If you cast the effect again on yourself while the effect is active, it does nothing; it does not stack. The intended uses are:
* Going from "comfortable" to "warm" temporarily in a warm place where you don't care so much about exposure.
* Bumping up to the next warmest exposure category temporarily
* Last-ditch effort; you can see the inn in the distance, but you're freezing to death and you know you'll die before you reach it.
- Alteration:
- Water Walk: A bit off-kilter to direct exposure consequence spells - water in Frostfall'd Skyrim is a major issue re: swimming. Why not consider a Water Walk spell? Decent reagent + magicka upkeep cost would make this a costly choice spell and one to consider carefully, and traversing long distances would be hugely dangerous if the spell gives out mid-way through. Similar to a heal spell, you have to channel it and move for it to work. I think it would be a great alternative for a mage, or for someone who doesn't want to bother gathering up snowberries and crafting an extract.
- Dragonhalt: (meh name, sorry) Kind of a top tier Skin Spell, similar to Dragonhide in relation to the 'Flesh' spells (following Skyrim's spell levels). For reagent + perhaps two-handed charge up, you gain immunity to exposure change and negative exposure effects for X seconds. At the end of the duration, however, you resume the status you previously were at on casting the spell. Best used, I feel, to help deal with a terrible combat situation in a blizzard, or a quick escape to somewhere you can warm up. 2-3 moderately expensive reagents + magicka cost. Reagents shouldn't be so expensive/rare as to make you hesitate using the spell in a dire situation, but should be enough that you don't really carry more than 1-3 casts at a time.
- Evaporation: This would be pretty neat, but balancing it to make it useful would be key. Having a very cheap reagent would be 'the go', I feel. Something cheaper than bothering to cast Conjure Fire-pit below. Perhaps make it channelled, so you have to hold it to dry off, or make it an over-time spell. Cast, done, but the effect takes a short amount of time to dry you off.
Water Walk was my favorite spell from Morrowind. I'm a little afraid of scope creep here. This sounds like a thing best served from a real magic mod. Let me think about it and perhaps do some research into who else has done this spell (if anyone).
Dragonhalt: Hmmmm. Not bad. Keep the risk there, but temporarily give their combat abilities back. I think that could work. I probably wouldn't charge a material component for it, but, we'll see. If it were sufficiently expensive and annoyingly long to cast, it would balance itself I think. I'm really trying to save the material components for the spells that really create "something from nothing", or have very big effects, and I think the shelter spells qualify for that.
Evaporation: On the one hand, I think a mage with billowing steam coming off of him as he channels a spell looks really awesome in my head. On the other, being wet is one of the riskiest things in the north. What about, only allowing you to dry off by 50% (Damp)? That would at least get you out of the Drenched and Wet penalties, without removing the risk entirely. I dunno, not sure yet. Or what about a channeled spell that dries the player, but also hurts them? (a la Equilibrium)
- Conjuration:
- Conjure Tent: As you've already got in your spreadsheet, but I feel your initial Conjure Shelter is a bit high-end, starting at 50, and as such I suggest a rank before it: Conjure Tent. You get nothing fancy, just a tent. Light reagent/magicka cost, perk cost as well to prevent everyone taking it and never looking at a normal tent again. One of the reasons I suggest a very simple Conjure Tent spell, is different situations. For a simple stop-to-warm-up situation might see you only wanting to use Conjure Tent, where as requiring a more major 'stop, sort inventory, craft some things, etc' break would see you casting Conjure Cottage or such. Having Conjure Tent as the lowest rank of Conjure Shelter spells would also have you, again, fall perfectly in line with Skyrim spell rankings of 1-2-3-Ultimate. (That's just neat, not a necessity.
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- Conjure Shelter, Lesser/Greater: These would be rank 2 and 3 of Conjure Shelter spells, identical to the 'Basic and Fine' spells you have in your spreadsheet, and the first to feature your lovely 'mage shell' design and invisibility.
- Conjure Cottage: Not ACTUALLY a Cottage, but similar in content. My initial thought (which you beat me to!) was based off of Leomund's Hidden Cottage, a 5th level Bard spell, essentially a superior Conjure Shelter that creates a nicely decked out place for its duration (which you've already got. XD). Basically a rename of the 'Exquisite' shelter you've got listed in your spreadsheet. This is obviously the top end 'rank' of the summon shelter spells, in-line with Dragonhide/Dragonhalt and such. Similar reagent costs to Dragonhalt: Such that you wouldn't carry more than 1-3 casts at a time simply by expensive nature/rarity of reagent cost. Maybe a little higher than Refined Malachite?
- Conjure Spriggan: Mid-level spell. Spriggan disappears to collect deadwood/firewood for you. Price would be mid-level reagent + magicka + time, perhaps slightly increased time as a trade off for the fact that you are not bound up in the animation/FTB. Would return with a mix of deadwood/firewood, then vanish.
- Bound Axe: Obvious, but this brings up interesting points on degradation versus magicka. See my discussion on it below.
- Conjure Fire-pit: a match to the Conjure Tent spell as it does not include a fire source. As I suggested in an earlier post, perhaps it could be conjured pre-lit, but said state only lasts for a very short time. It would be enough incentive to cast it if you're freezing to death, and last just long enough to stave off death, and then possibly toss in excess clothes and books to keep it going long enough for you to get moving again, possibly allowing you to make it out of wherever you are alive. The length in which it is conjured lit should be long enough to give you a few measly exposure points, and not much more. A reagent cost would seal the deal on it not being something you can constantly re-summon for free exposure. Should dispel on two conditions: if unfed, the unlit fire-pit should dispel after X hours, giving the player time to find fuel for it. If fuelled after summon, it should then dispel after it goes out. Not sure how cooking is handled with fires you make in Frostfall (sorry, bit derp of me not to know!) I understand a cooking pot is required, so that would make a standalone conjurable fire-pit useful on its own if you don't intend to sleep, but rather simply to warm up and maybe cook some things.
- Summon Luggage: I've always wanted to see a spell like this. There are a number of 'Summon Storage' mods around, and I noticed your Conjure Shelter - Fine/Exquisite have storage chests, which I assume are 'cloud storage', in that re-conjuring the shelters and accessing the storage, it will have the same contents as you last left it. Love the idea, would like to see maybe a greater 'spread' over the ranks? Maybe just a sack in Conjure Shelter, Lesser, a chest in Conjure Shelter, Greater, and then a chest, sack and maybe an alchemical satchel in Conjure Cottage. More containers simply allowing for some storage sorting (for those of us with roleplaying tendancies and/or OCD (I have booooth! XD)). However, the idea I keep thinking of is (if anyone's read any Rincewind/Discworld stories they already know where I'm going), is a summon-able perma-storage 'luggage', a chest on 'feet' or floating that follows you around when summoned, and vanishes at the end of its appointed time. Not sure how it would be done, but http://img843.imageshack.us/img843/8228/theluggage.jpg There is a D&D spell called Tenser's Floating Disc that had similar properties. It could carry X pounds, floated only a few feet from the ground, and could serve a number of useful purposes. Loved having it around (and when empty the party gnome would ride around on it). Not sure how well this general concept would fit into Frostfall, but as previously stated, throwing ideas at the wall, maybe it'll stick. Mages don't want gaudy philistine-looking backpacks, right? ThMaybe they'd rather have a floating chest familiar carrying their unimportant gear while their main inventory is taken up with their survival gear...
I thought Conjure Tent would be boring, but maybe you're right in that there needs to be a lower-end option starting at 25 for a budding mage. My problem with it is someone teching up to 25 conjuration relatively quickly just for that spell and then going through the rest of the game never needing to carry a tent again. My definition of "serious mage" starts at about 50, anything less I consider amateur or casual spellsword without a serious magic commitment. The barrier to entry to circumvent the need to carry a tent I feel is probably too low at 25. It would need to be inferior to a normal tent. Especially in light of the fact that the Shelter spells are essentially a combination Fur + Leather tent, in that they keep you completely warm and dry; I think the high spell skill requirement and the material components offset this however.
I want to keep the number of possible shelters in check. Too many and things get confusing, and people wouldn't use half of them. I'd rather refine the ideas around a few very good choices than add additional ones that might not get much use.
Conjure Spriggan might as well be "summon firewood" with additional flavor, so we have to look at it in that context. The question is whether or not I would want such a spell, and the answer is generally no. I might also have a player expectation issue with this ("Why can't I keep it as a companion and let it fight with me?" "My spriggan attacked the jarl and now I'm homeless..." "I found my spriggan and Ysolda together, is that bad? Should I be worried?"). However, with Transmute Wood, I guess I sort-of already have a summon firewood spell. You just have to jump through more hoops to get it. Harvest Wood, Deadwood, pick up deadwood, transmute wood. Is that more fun or less fun than a spriggan that does it for you? I'm not sure. I'll seriously think about it, it might be cool.
Heh, Bound Axe. It's logical but I'm trying to keep things a little separated there. I'm already taking some artistic liberties with the bound cloaks; according to canon, conjuration only summons things directly "from Oblivion". So daedric-looking armor and weapons. Granted, Oblivion is just a type of plane, of which there are many, but still, I'm trying to keep that usage under control. Also the gameplay implications of that are not so hot.
Conjure fire-pit - Yea, the time and pacing and requirements of this would have to be carefully handled. It veers precariously close to "magicka-for-exposure-loss".
Cloud Storage LOL. I guess that's a very accurate anology.

Any sort of thing that follows the player is prone to failure (getting stuck, so on) and is something I want to avoid for the sake of my own sanity. But a summonable chest is fine, as long as it costs you something material to access it.
Going ahead and posting this so I don't lose it.