Let's be honest; the Snow Elves actually have a hell of a lot more in common with the "Drow" of Forgotten Realms than even the actual Dark Elves in the Elder Scrolls universe. All over Skyrim, it's constantly alluded that they're this incredibly badass race, yet thus far, we've seen little more than unintelligent savages. In Dawnguard, however, we get out first real glimpse of something bigger...
What if there were a greater, underground society, unbeknownst to anybody? Not just a band of survivors, but a glorious, sprawling city, stretched out as far as the eye (or draw-distance) can see? A place where, instead of fighting trolls and dragons, you face deadly assassins, bandits, and corrupt guards within the city itself?
We know damned little of the Snow Elves, but we do know they were a proper "beautiful" race, once. After their enslavement by the Dwemer, they became unimaginably cruel, but once they eventually rebelled, it's entirely possible that they could've gathered, en masse, to build a massive underground city. Like the Drow/Dark Elves of the Forgotten Realms, though, this society would be even more secretive and brutal than the Falmer we've already encountered.
By thinking is that, in contrast to the rest of Skyrim, you would eschew traditional Fantasy in favor of more "political intrigue". Instead of a city-hub with various "wilderness" segments, I'd like to see it made up of one massive city, but with varying sections. Certain, low-class sections would be rife with thieves who might randomly steal gold or non-worn items from you, or common thugs might follow you and attempt to attack you when you enter a particularly shadowy area. In the higher-class sections, you might even get crooked guards who try to extort money, beat you randomly (and charging you for a crime if you fight back), or even make demands for Weapons and Armor you're currently wearing.
While that might sound crappy, it would be off-set by the gorgeous underground caverns, which we've already seen can be quite beautiful. Perhaps taking it a step further, however, maybe they would utilize intricate designs throughout their architecture, something wholly new and unique, with several of the important families within the city each having their own keep, lavishly decorated and brightly lit. Maybe they would even utilize lots of stained-glass and glowing flora in their architecture, with immaculately decorates Balls and social-gatherings (something similar, perhaps, to the Mistborn Trilogy, if you've read them).
In addition to a whole new set of Quests, I would also make the Snow Elves a fully-playable Race. For their Racial abilities, I'm not really sure where you would go; I suppose it depends on how you choose to develop their culture. Since the Khajiit already have both Night-eye and Clawed-fists, perhaps they could get a built-in Detect Life, or even a Power which makes everyone nearby immediately made blind. I quite like the thought of them being more powerful at night and less powerful during the day (which would make them *AWESOME* Vampires), but that might be a little too nuanced for a Race. Either way, definitely make it something more "practical magic" moreso than "arcane magic".
Beyond that, I would love to see the Morag Tong given a presence, perhaps allowing you to advance more quickly if you've proven yourself in the Dark Brotherhood or Thieves Guild. In addition, I could see them having their own "Mage Guild" of sorts, perhaps as practitioners of Conjuration and Illusion, maybe even allowing you to summon full-fleshed Daedra beings? And for the honorable Warrior or Fighter types, perhaps they have their own Colosseum? If so, *PLEASE* disable the Save option while participating, and allow us to lose matches without dying.
In terms of factions, beyond my personal interest in the Morag Tong, you could absolutely bring back the idea of Great Houses, with perhaps some sort of "Council" type of organization at the helm. In line with the character in Dawnguard, perhaps there are different Fighter organizations, ranging from Archer-types (ie. "Stalkers"), Dark Knight-types (ie. "Spellswords" or "Nightblades"), and the "rogue" faction that seeks to do good, the Paladins (for those unaware, the Snow Elf NPC in Dawnguard is references as a "Knight-Paladin"). As well, maybe even House/Family has its own sort of Faction aligns with it, obviously restricting you to only aiding one or two in a single player-through. In doing so, however, perhaps you could be awarded the Keep of a rival House, depending which family you aid?
In terms of Gameplay functionality, I ***REALLY*** want to see Archery and Dual-Wielding amped-up a bit. For Dual-Wielding, some ideas would include being able to Roll or Dodge in combat, instead of a basic Sprint, and also the ability to attack several people around you. Also, if you stand still and press the Sprint button, maybe your character does a brief "Parry" animation. If an enemy hits you during the brief animation, you counterattacking for greater damage, but if you Parry too soon, *their* physical attack will gain the bonus (would not affect Spell damage). In terms of Archery, let us affix different attributes to our arrows, not only to increase damage via Poisons or Flaming-arrows, but also other more nuanced things, such as Flares or even "Weighted Arrows" which cause nearby enemies to walk SPECIFICALLY to our arrow to "investigate the noise".
Also, among other Pekrs, I'd like to suggest something specifically; "Whisper". Basically, let us Cast spells and Shouts whilst Sneaking, without nearby targets immediately being notified of our presence. This mostly applies to Self-buffs, but it could be extremely useful with the "Become Ethereal" Shout, as well as the aforementioned Arrow-augments which would likely be done via Spell-casting.
And finally, the good stuff; Weapons and Armor. In terms of weapons, I would absolutely add Scimitars, however I would also add a myriad of gaudy, expensive/priceless looking items. Maybe the Daggers' hilts are made from solid gold, with radiant jewels adorning them, and bright, gleaming platinum blades? For Robes and Clothes, maybe they lean towards the like of Cleopatra with lots of exposed skin, while Light Armor would all be tight-fitting leather with more "deadly" looking designs. For Heavy Armor, there's really no precedent, though personally I would love some over-the-top, "Death Knight" styled armor, perhaps also incorporating long, regal capes. Conversely, the Paladin Order (whatever its name may be) might offer slightly more "Divine" styled armor, also playing up the Polished-Gold aesthetic, but making those Armor and Weapon types more of a minority in this expansion.
The idea is simply to offer something wholly unique from Skyrim proper. The fact is, even though Riften is home to the Thieves' Guild, it feels like a game about big manly-men doing big manly-men things. By delving into the Snow Elves and a society that places more emphasis on politics than honor or physical-might, it creates a haven for those who enjoy playing Thief and Assassin-type characters, while at the same time providing something for Mage and Warrior types, as well. We've already got an amazing game that lets us play the Noble Hero, so what about casting us in an underground world where the city-streets are your battlefield?