Your mileage may vary
1) I want simple, drop-in / drop-out co-op capability. I like ESO but frankly I never really needed to play with tens of thousands of people. I have always enjoyed the single-player TES games more but I have also wished I could just co-op it with a friend or 2, much as I do Borderlands. I don't need deathmatches, I don't need a hundred people hopping around at the bank calling people noobs, I just want to play alone or with a couple of friends.
2) I want better replayability. I have over 300 hours logged into Skyrim but I find it hard to replay again. I don't want to re-do so many quest chains just to be functional (like having to re-do all of the Greybeards quest line to get my shouts in order). I want to make a new character, pick a direction, and have a fairly unique gameplay. No, I don't need everything to be generic and yes I do like overarching main plot lines, but I am confident bethesda can include such things as well as sufficiently randomize sections and locations of the game without compromising quality. The urge to make a new character is very strong but the repetitive similarities are daunting.
3) I want better looking characters without resorting to mods. I felt there was reasonable progress on character appearance and customizability in ESO over Skyrim but I still dislike many of the base models, particularly elves. Our characters are our primary foundation in the game, let us look amazing.
4) A larger, more diverse world. What can I say, I can't get enough! And the chance to cross a wide variety of environments can only enhance the experience of being in your world.
5) Magic system, magic system, magic system. I miss the Oblivion days of custom spells and discovery. I want it back and in a bigger, better, more detailed way. I don't necessarily need to become the head of the mages College again; I just want to study there or cobble together my own spells in a richly detailed way. I want to feel like a mage.
Frankly, that's it for requests. I thought Skyrim was one of the very finest RPGs I have ever played and am extremely confident that the next installment will be amazing, regardless of whether or not the studio listens to my suggestions. For me, though, a new TES game with these components will blow away every previous version and go down in gaming history as the rolls royce of RPGs. Again, your mileage may vary and please, while you may disagree here and there, I really don't want to hear the excuses I once heard from other players about how developers haven't time to implement good things or how no one will ever make a co-op ES game when you don't write their schedules and we now have an ES MMORPG. Anything is possible, they just have to want to do it.