Dude, it's games like yours that make me fall in love with games all over again.
So, far I am having a great time just messing around building gigantic structures.
I can't wait until the dungeon mode is implemented that idea is pure gold. My thinking about that mode is that you should allow the creator of the dungeon to edit the strength and weaknesses of the monsters as well as the attributes of the spawned items and gear. I would also love it if there were some kind of creature editor so people could make really crazy bosses and minions (think of creating a 30 block tall pixely balrog).
In regards to the team modes I believe that this game would benefit from a Team Fortress class type of setup (which I believe has been briefly touched on in this thread) in which two teams guard a valuable artifact. There would be builders (to build cleverly constrcuted mazes, hidden rooms and pitfalls), stealthy spy types (to scout out fortress to see the layout/defenses and to try and sneak past traps to get to the artifact), defenders (to defend the artifact) dungeoneers (would place traps to keep opposing team's players from getting to the artifact and would go with groups of players trying to gain entry into the other team's fortress to disable their traps) and miners (to harvest resources for building structures and traps).
Basically a team would try to gain entry into the other team's massive trap filled fortress to obtain the artifact and bring it back to their fortress whereby a timer would countdown until they one the game after a certain duration of possession. In the beginning of the game miners would scout the area for material rich sites that would then be under close guard by defenders to secure their team's ability to build.
The only class I am not quite sure how it would work would be the defender since I think that the team capture the flag mode mode shouldn't have any direct combat because I believe it would keep it more interesting with the whole idea of traps and such (and direct combat could be used for deathmatch or maybe there could be two different types of team flag modes, like say a team-flag and a team flag-frag).
Also, now that I am going idea crazy here I think that perhaps the "fortress mode" coould be a cooperative tower defense game where you and your team have to defend the keep from a relentless onslaught of monsters (which in fact could be peppered with other players Left 4 Dead style). The team would have a limited amount of time to quickly build whatever traps, turrets, monsters/minions to defend with and the difficulty would gradually ramp up as you had to manage repairing parts of your castle that get destroyed and defeating monsters that get inside.
Sorry for the crazy long post but this game is awesome
I can't wait to see all of the great ideas in this thread implemented.
Keep up the good work
*edit* btw I think I may have an interesting idea fro the defender class I mentioned, perhaps they could be the ones to use some sort of materials to create monsters to defend their respective team's fortresses? I don't know just a thought.
also, I think that builders should have the ability to build more external defenses (as opposed to the dungeoneer's traps) such as turrets/catapults and such to give the class more versatility then just building structures).
p.s. I love the idea of posting scores on some sort of online leaderboard showing how fast, how many points and such a certain player/team complete a map so as to have something to compete against and also giving maps more replay value.