Don't know how these ideas would be in a game, but what I'd like :
1) Dynamic World. Once you kill King Dain Coldreaver 4, he is no longer there, ever. Either your team killed the whole coldain population and they are now extinct, either a new king is put on the throne.
2) Dynmiac world implies that your character is permanently in the world, even if you are not playing, meaning that you must put him in a safe spot such as an inn in a good city
3) Full PVP, lootable corpse, bounty, npc/pc investigation
4) 4 hit points : bruised, wounded, badly wounded, severed. No in combat heal spells. You must use protection to survive.
5) No classes - if you want to become one, you go the temple of Marr and then you go through the trial to become one.
6) Ships
7) huge world
8) endgame is not "everybody hits autoattack on the dragon, move out of the fire, click the gems, jump over the fire circle, kill the add". It must feel like storming a castle, ie from when you step in Permafrost keep, Lady vox starts to throw shit at you. Tons of small fights, etc.
9) UO style of skills.
10) Faction : you cannot go in the king room and hump him till your fingers are sore - you must first gain reputation.
11) Fun tradeskills, with entirely customizable design, imbue, gems, runes, etc.
12) Housing that can be looted if you're not carefull.
13) Seasons. I was half hoping that WoW would implement that. Imagine Westfall during summer, winter, springs, etc. would have been gorgeous and a sense that time is really going on.
14) a very complex magic system that starts simple. Combine voice, runes, ingredients, gems, place, time, phase of the moon(s), number of people that helps you channelling, etc. I'm no theorycrafter, but I want them to commit suicide over this system.
Personal taste of world setting : I've always loved "End of time" settings such as Elric the necromancer or Cugel the Clever. I'd like to see an Everquest 3 game taking place like 400 years after PoP; a few days after Quarm was defeated, magic slowly dries out until no spells can be cast again. 2/3 of the population did of starvation, madness, war, etc. Then one day, children are born with a mark and they can cast magic. Player would only play caster (not meaning that they must only be mages, they could use magic-enhanced sword skills, etc)