I don't know if this has been suggested but I'd like to see an "Infinite" mode. No end and no progression, just get through as many levels as you can without being killed. Maybe allow players to select which level "theme" to use (doubles as a sort of difficulty selector).

Doom Raider

Requires ZDoom.

Suggestions (sorry if these have been suggested before or are already in-game. Been kinda busy):

User personality traits: People play MMOs for different reasons. Explorers like to explore and fill in maps, so having a large world with lots to see keeps them playing. There are players interested more in PvP. There are players that love collecting things (loot, skills and abilities, ranks/titles). Perhaps these can be added to the simulation so your MMO might cater to one kind or you can try strike a balance to keep more people around.

Ability to set drop rates: Everyone loves loot. What if certain zones could have their loot drop percentage set? You could have Common, Rare, and Magic (or whatever) loot qualities. 80% chance to drop Common, 15% Rare, and 5% Magic for a typical zone. If you let the high end item drops too frequently they become worthless and not worth seeking out for Users. If they don't drop at all Users get frustrated and quit. Maybe you can create a high level area near some low level ones that has a better chance at dropping Magic items to lure players into a challenge?

Keep up the great work! This is a really cool concept.