As a long time MMO player I've given a lot of thought to the kind of MMO I'd like to play, and after listening to your segment on MMOs this week I decided to throw it up here.
I'd like an MMO set in the Game of Thrones world, where the objective was less focused on theme-park events like dungeons and raids, and more on the kind of political conflict over territory that takes place in the books. Players could chose to represent one of a number of factions and fight over land, castles, and titles. The most dedicated players and guilds could rise to be the kings and queens of their respective faction, giving them control over tax rates, war declarations, treaties, hiring NPC guards and merchants, and other powers. Smaller guilds could control smaller land holdings, keeps, towers, and villages, with the key being that not every guild is it's own faction, but numerous guilds belonging to one of several huge factions. Castles, keeps, and land could all be conquered and taken by another faction through Aion style timed sieges.
Rather than just being an entirely PVP focused game, there would be many elements to running a kingdom that would require the help of PVE focused players. For example, repairing castles damaged in sieges, equipping NPC guards and armies, and feeding those same NPCs would require a large player base to obtain the supplies. In this way even lower level or peace loving players could contribute to their faction (and make money) through crafting, mining, fishing, farming, or adventuring for materials. In return the big guilds who ran the war efforts of the faction would have an incentive to protect the "peons" of the realm, lest they defect to a faction that would treat them better.
For the combat I'd like an action combat style, similar to TERA but with more precise control. Something like the combat in Chivalry: Medieval Warfare, but with some special attacks like TERAs to keep things interesting. Finding some semblance of PVP balance would be key, while keeping all the classes fun to play and viable in PVE as well.
By establishing those as the base setting for the game you'd have conflicts in place to keep the hardcore crowd, who tend to burn through game content, active and playing. From there start adding variety in the form of more traditional theme-park style events: dragons like TERA BAMs to fight, group and raid dungeons, lengthy multi-step quests (think Everquests 'epic' quests), ship to ship sea battles, etc.