Cooperative would be very nice - but I can appreciate the anti-socials fretting about other people having that choice.
Mostly, I want expanded mechanics and I'd love a much more elaborate stealth/thievery system.
I prefer the new spell system - but it could use a lot more variety.
Balance was always a joke - and I'd really like for them to implement an "old-school" mode with zero level scaling.
I'd like crafting to be vastly expanded as well, and I don't want character power progression involved with that. I'd love the ability to create and customize more stuff, including furniture and even a complete house. But more customization is key.
I'd like MUCH fewer dungeons with MUCH more variety. Say, perhaps 50 dungeons instead of hundreds - and all of them completely unique and most of them very large.
If they're going to go with randomized quests again, I'd like for them to be more convincing as something that could actually be a real handcrafted quest.
I'd like for them to work harder on the crime/reputation system. Make it a real challenge to be a good thief or a criminal in general - and make it much less exploitable and gamey.
I'd like for them to not rely on the community for a good user interface.
I'd like to see a new engine that streams content properly - and which doesn't have the severe flaws of the "Creation" engine.
I'd like to see water and oceans play a larger part - including the ability to sail ships and much more elaborate underwater exploration.
I'd like puzzles and riddles that take more than a 5-year old to solve them.
I'd like to see a fast travel system that's less extreme and less convenient - with more interactivity and immersion (like an actual transportation system). Make it cost something and use a wayshrine system similar to ESO. It's simply too convenient and I don't have the discipline to ignore it at all times.
I'd like to see magic and stealth work together.
I'd like to see more tactical melee combat - with a more deliberate pace ala Dark Souls.
I'd like to see materials mean more than simplistic damage increases. I'd like elven weapons to be visibly faster and with a higher crit rate, and dwarven weapons hit really hard - and their armors be heavier and less flexible.
I'd like to see the enchantment system expanded with many more prefixes and suffixes.
I'd like to see the unique artifacts be much more powerful - but also much harder to find, which would work in a non-scaled environment.
I would like to see a much more balanced and dynamic economy - where your actions directly influenced supply/demand - and where gold is much more useful and much harder to come by.
I'd like the ability to take over and build cities - and eventually run them and setup your command and administration staff.
I'd like a more elaborate hunting system - akin to Gothic 3/Witcher and beyond. Especially in the early levels - I'd love for skinning and taking trophies being a bigger deal.
I'd like the main story to be 2-3 times larger and less "mainstream" explosion based - as in more subtle and with more C&C.
I'd like each of the Guild questlines be around 30-40 hours - instead of ~10 hours.
I'd like to see exclusivity in the guilds again - and I'd like every rank to be much more meaningful within each guild - and I'd like for the last few ranks to be excessively hard to achieve.
I'd like to see more survivalist aspects ala Fallout NV and the mods for Skyrim - but make them optional and implement hunting and such to match these features.
That should be enough to start with