What sucks?
What would I want?
Very broad questions based on a very touchy subject. You might want to break these things down and give us a little more of an idea of the framework you're setting here.
Anyhow, here are a few things.
-PvP has been a backseat afterthought in most of the games we've played since SB. This, in and of itself, is a factor that makes most titles lack in that dept.
-Individual skill has really been drowned out in most MMO titles and the whole class based Rock/Paper/Scissor thing is getting extremely overdone.
-At what point do future MMO developers stop trying to reinvent the wheel and make use of existing technology that is proven? Too many titles have been a major disappointment as a result of this due to falling off of the timeline and then rushing an unfinished, untested game to store shelves.
-Marketing to EVERYONE. Every MMO wants to beat WoW, emmulate WoW or compete for WoW's buck. Now is the time to grab niche markets by making games that have a specific vision. Developers should make no apologies or compromise when they feel their vision is in place. Make the game you'd like to play.
-PvP as a primary focus in the game mechanics, economy and ruleset.
-Lack of a "candy land" approach where games are developed to make everyone feel like the hero and that everyone is special. You should have to earn your reputation in whatever facet you wish to dominate, be it a trader, PvP'er, hardcore ganker or big time Anti-PK.
-UO was the only game that I really felt that skill was important above equipment. We want to see a new game provide an intuitive approach to making everyone on the field be potentially deadly instead of class based shit.
-I'm sick of seeing developers go heavy handed everytime something needs tweaking. Most often, its as if they pick up a battle axe to do the job that requires a scalpel. All too often balance is lost as a result and more importantly, people's valuable time is wasted.
-Devs should be careful when reading their own forums. People whine there. People cry for attention there. All too often, Devs fall prey to the whims of their forums instead of making the game they started with.
-Let the playerbase govern themselves as much as possible. If you make a solid game, let the game mechanics handle issues. Only time GM's/Devs should step in is when the mechanics are exploited, they break down or the system needs minor tweaking.
-LOSS AND REWARD!!!! I want to earn loot and fame from my kills/conquests. Death should MEAN SOMETHING.
Any future PVP game that doesn't set a high mark for excellence is only going to hurt the genre, not help it. I can look back to games like Shadowbane and point out that the game itself was mediocre, not the ideas behind it. Can you imagine SB if it truly worked as intended and was a stable game?
That is more than I set out to write, but there you go.