Wow, I go get some sleep and when I get back, so many new replies...
Well, lets start making a list of suggestions:
-Empire building kind of game using FAF (frost and flame) as the base engine, or made from scratch
-Adventure game using Dragon Knights as base engine, or made from scratch
-Using Eclipse engine to make more of a dungeon crawl game (not really browser based since it needs a client, but still good enough)
-Cooperative fortress building, with traditional 2D top view with tiles
-Qwertyuiopas' deviant of the fortress building, with a more general grouping of tiles and buildings
-And the ULTIMATE goal (in my opinion), the combination of all the aspects of DF as Servant Corps proposed, as in the choice to be adventurer or fortress leader, and have these different playstyles interract in a consistent way in a consistent world. This should probably be classified into a very long term goal, we should start with one play style and then expand into the other gradually.
As for how to actually make the game, it definitely depends on wether we want it to be a real time game with graphics (even if minimal), or a text based and tick-based more traditional browser game. If we go with the real-time, I think Eclpise is the best choice. If more traditional browser based, either use one of the afformentioned engines, or PHP or ASP to make ours from scratch.
(I'll put all this in a poll now, tell me if I forgot any other suggestion)
EDIT: I forgot to mention, I'm in favor of using the Eclipse engine, and for the game to turn out like Hawkfrost said, like FF1-3 and similar games. I really like these games