After looking up many class guides, I see that many of them have instructions but little to no reasoning behind them explained, and many also pick things that may not be necessary or rely on very strict structures. I make this guide in hope that it will help provide some relief to those who want an easier time planning their characters' perks and stats.
This guide visualizes each skill tree as a "module" to the character, and will focus on the bare-bones minimums for character types, so players will have lots of points left over.
Try this skill calculator for some help in planning: http://skyrimcalculator.com/
Comprehensive, Modular Skyrim Class-Building Guide:
Vitals:
Stamina - A high stamina is mostly important for those who are going to be spending a lot of it on heavy attacks and making up for heavy armor. Characters who favor speed and less intensive activity will need less stamina. However, even heavyweight characters may not necessarily need much either thanks to the Heavy Armor perk that makes the armor weigh nothing. In the end, this will be all about how often you want to do power attacks. 200 is a very comfortable amount for most character types.
Magicka - Your level of magicka depends on how much you want to enchant your gear, and how many perks you care to spend. There are 5 magic perk trees, each with 5 levels of mastery to cut magic costs in half. The highest Magicka cost for a single spell is 1025, while all other Master spells average around 900 or so. With perks, the average is around 450. With perks and an enchanted necklace+ring, this can lower to 0 for two schools, or ~225 for four schools.
Non-magical characters can ignore Magicka altogether, but those wanting to be good at magic will need a fair bit of it while leveling up. 400 or so can be nice for a full-fledged Mage who isn't exploiting, but even without exploits this number can be rendered obsolete. 200 or so might be better for heavy enchanters, or those characters who just want a little magical skill on the side.
Health - Everyone needs a lot of this. Generally, most characters can add Health and Stamina at a 1:1 rate or so until Stamina is at the desired level, then the same for Magicka, then dump everything else into Health. Mages might do this in reverse, or even at a 1:1:1 rate. Ultimately, it just comes down to preference while leveling up.
Skills/Perks:
Each skill will be listed with a number range that represents the number of must-haves to the recommended amount for a character that specializes in that skill.
Crafting:
Alchemy (10-14): Can be fairly useful to those who choose to use it, and can also be heavily abused, though spending perks isn't necessary for this. Fair players will want 5/5 Alchemist, and will likely want to follow one of the paths (poisons or potions) up to Purity.
Enchanting (8): Enchanting can be very abused to make all spells cost no magic, or weapons do infinitely powerful damage. Still, even fair players will probably want to take 5/5 Enchanter, Insightful and Corpus Enchanter, and Extra Effect. This will let you improve all the abilities you like. Soul Siphon and Soul Squeezer can very convenient for adventuring, but not necessary.
Smithing (2-10): In the game, you will find all the craftable pieces of armor anyway, and you will reach the 80% armor cap with only 567 or 135 with perks. This entire tree is basically to improve your weapon without abusing Enchanting. Arcane Blacksmith is necessary to improve magical items at all, and it requires Steel Smithing.
Weapons:
One-Handed Weapons (6-10): 5/5 Armsman is necessary for good damage. Fighting Stance and Savage Strike are great if you want to do good damage. (If you're not into decapitation, Savage Strike can be ignored.) The dual-wielding perks are great for anyone who wants them. Specific weapon perks are very weak, and everything else is situational.
Two-Handed Weapons (8-9): 5/5 Barbarian is necessary for this tree. You will also want Champion's Stance and Devastating Blow for damage, and Sweep to damage many enemies at a time. Specific weapon perks are very weak. Since you only have the weapon and no spells or shield, the other perks may be useful.
Archery (5-14): 5/5 Overdraw is necessary. All the other perks (except Critical Shot) are very useful for all characters who use bows extensively.
Defense:
Block (8): This may not be necessary, but it is a very useful tree to invest in if you use a shield at all. 1/5 Shield Wall (20% more effective) is the most important and all the others are optional, but everything except Quick Reflexes is very, very useful. The other 4 points of Shield Wall will only get it up to 40% more effective; 4 perks for very little benefit.
Light Armor (5-9): 5/5 Agile Defender is necessary for any light-armor-wearers. The other perks are optional, but Wind Walker can make your stamina 50% more useful than it already is. That's a lot of extra power attacks.
Heavy Armor (5-12): 5/5 Juggernaut for sure. Other perks are very useful but not necessary. All fist-fighters will want Fists of Steel to do any real damage, and it's a good fall-back since an unarmed warrior will have few other options.
Alteration (10-14): The magical defense tree, this can provide a solid defense against or via magic. Remember, there is no penalty for a mage wearing armor in Skyrim. But if you choose not to use armor at all, Mage Armor is a great choice. All the other perks have great use as well, and given that most of the tree relies on the line of Alteration mastery, that last point in Master Alteration could be worth it. This tree is more-or-less an all-or-nothing tree.
Spells:
-All Magic Skills-: Every magic skill, including Alteration, has a "mastery" line of perks that cuts different levels' spells' magicka cost. If you are willing to put in the work into Enchanting, you can avoid almost all of these. If you don't plan on getting a lot out of Enchanting, these will be necessary for any magic trees you delve into. It is recommended to get the suggested Enchanting perks if you plan to use magic at all.
Illusion (4-8): A dedicated Illusionist will want both sides of the circle leading up to and including Master of the Mind. Any stealthy spellcasters will benefit greatly from Quiet Casting, which requires a few points leading up to it.
Conjuration (2-6): The important one here for any summoners is Twin Souls. Pick the side (Atronachs/Summons or Undead) that suits you, and get up to it. With the master-level spells (which can cost over 900 magicka), this will allow for two permanent summons. For those who prefer bound weapons, Mystic Binding is useful with subsequent perks not necessary. Anyone who uses Bound Weapons will benefit greatly from the associated weapon perks.
Destruction (5-13): Impact and any Augmented Element perks are must-haves for damage and control. Intense Flames can be counter-productive at times.
Restoration (4-5): Recovery is almost necessary for any dedicated spellcasters, and for only one more point there is the incredibly-helpful Avoid Death. Actual healers mostly just need Regeneration.
Utilities:
Sneak (2-9): 1/5 Stealth (20% better) and Muffled Movement necessary for anyone who wants to be good at sneaking. The other 4/5 Stealth only brings it up to 40% better; 4 perks for very little benefit. All the other perks along the Muffled Movement line are also very, very useful. Get the sneak-attack improvements as they apply to your character's other skills.
Lockpicking (6) : If you choose to specialize in Lockpicking at all, the important skill is Unbreakable. Master Lockpicking is a waste at that point.
Pickpocket (5-9): 5/5 Light Fingers is necessary for anyone who wants to be good at using this tree or the very-situational pickpocketing in general. Later perks make this tree much less situational. Characters will also want to have the minimum suggested Sneak perks.
Speech (6): A purely optional tree. Spending the 6 perks necessary for Master Trader can make your money-making endeavors very easy. With a high Speech level, Persuasion and Intimidation may not be very beneficial.
Class Archetypes
Here are a few class archetypes listed with the bare number of perks required to define the class, as well as an example of what the character's (Magicka/Health/Stamina) might look like when maxed-out. (All spellcasting assumes mastery trees are taken.)
Remember that the point of all this is to view each skill as a module in your character's overall feeling, and getting the minimum number necessary to accomplish that goal. As such, these are made to be very open for expansion, mixing, and matching.
Assassin (31) (100/800/200): Alchemy 10, Stealth 8, One-Handed 7, Archery 6
Warrior (25) (100/750/250): Heavy Armor 10, One-Handed 7, Block 8
Barbarian (20) (100/750/250): Heavy Armor 10, Two-Handed 10
Healer (25) (450/450/200): Restoration 11, Alteration 14
Mage (39) (400,500,200): Destruction 13, Restoration 4, Alteration 14, Enchanting 8
Necromancer (13) (400/500/200): Conjuration 9, Restoration 4
Nightblade (22) (300/600/200): Illusion 4, Light Armor 5, One-Handed 7, Sneak 6
Thief (26) (100/700/300): Sneak 6, Lockpicking 6, Speech 6, Pickpocket 8
Conclusion
When planning your character, remember that you can accomplish a lot with very little. Many perks may seem convenient, but very few are actually necessary to make a character feel "right", leaving you with many spare ones to do what you like with. Suggestions for this guide are very welcome!
What sorts of classes would you make? How far can you reduce it, or even the ones I've listed? What is your character, when boiled down to its base components?