Hmm probably somthing like, new engine that supports better systems for lighting, seriously my main complaint about skyrim is it's too bright everywhere, even with mods to make it darker i still felt it was far too bright.
Better combat overall, i want somthing more akin to dark souls, where the weapons actualy handle differently and just spamming away with basic attacks would get you killed. Likewise better more varied magic would be nice, rather than three elemental spells that all do the same thing but slightly differently. Obviously more balanced stealth and archery would be nice since in skyrim if your a stealth archer you basicly cheese the whole game. (when i say like dark souls, i don't mean from the punishing challenge aspect, i mean having moveset variance across weapon types)
More item types, i want spears, halberds, axes, swords, daggers, maces, throwing weapons, bows long and short, crossbows, flails, bucklers, kite shields, round shields, tower shields, the list goes on, basicly if it exists as a medieval style weapon i want it in the game. The same goes for armor aswell. i want more than just helm, chest, boots and gloves, i want belts, capes, shoulders, leggings and anything else that could be conceivably added, including under armor, such as a leather brace under a chainmail shirt.
I'd also like to see stats make a return, just having health, stamina and magicka realy hurt the game i felt. Keep the perks system but rework it so that basic functions such as shield slam stagger are built in already (based on the type of shield you use vs the armor of enemey, or somthing simular) get rid of all the boring damage/effect percentage up perks, replace them with fun ones like proc chance for status effects, example firebolt damage scales with stats but perks give it chance to explode for area damage, chance to cast a secondary spell for free, chance to summon an lesser atronoch, basicly whatever floats your boat for additional effects, melee would ofcourse have a simular system of prc effects.
I'd love to see loot be more appealing aswell, one of the biggest issues with skyrim is that when you get say a skyforge weapon, it basicly outclasses every other weapon type you'll find in game for a long time, when all the bandits and draugrs just drop iron/steel or it's equivilant, because so few of the items come pre-enchanted, there is basicly no reason or opportunity to swap out gear untill late game. I'd love to see random prefixes on items, kinda diablo style with various rarities, from basic ones to that give a few stat points to more legendary items that have a whole range of effects. Unique items with matching set bonuses would be nice aswell.
If i think of anymore will wishful requests i'll edit this line and add them in.