Edit: Posted this before reading about your teams idea, disregard if it doesn't fit.
Very fun game, regarding the first version. I had never seen it before, but what I did play of it was pretty fun and enjoyable. It was a little hard to find the dungeons, so maybe an overworld compass could be a purchased item available.
As for having other players visible, that would be a great idea. Here are a couple of solutions (Maybe not the best, I am not the best programmer) to the issue of syncing monsters. First the harder one, then the possibly easier one. Well, it's going to be hard either way, but here they are:
The first person in each room is the "host" who sends the position of the monsters to the other players in the room. When that player leaves the room, the next player in line becomes "host" and sends the position to others in the same room.
Pros: Multiplayer (which seems like a necessary feature for this game type), synced enemies
Cons: Space(number of players in a map or dungeon can ruin the experience of fighting), might be hard to code, might desync or cause lag, "Host" player could cheat the location of enemies locally, "hosts" with laggy connections
Have a hub town where all the players can see eachother, and form a party of up to three or four. Outside the town, you only see yourself, and if applicable your party members. The party's "host" will send data regarding enemy locations. If that player quits or leaves the party, they can play solo, and the next available party member will send the location of enemies to the rest of the party. You may have to limit party members to the same screen, sortof like what Wanderlust does.
Pros: Better on space, Co-op multiplayer, synced enemies, need only send data between party members on the same screen and not everybody else in the game, perhaps less lag, seems
easier to code?
Cons: "Host" player could cheat the location of enemies locally (though not as bad of a problem if players can leave party, and since it's co-op), "hosts" with laggy connections, may need to limit players to the same screen a la Wanderlust style depending on how your system works
Personally, I like the second method best. Again, they aren't the best solutions, but they should work well enough for something that isn't a AAA MMO
If anyone has any better solutions, feel free to chime in.
Another suggestion, chat is kindof iffy in these types of games. It can either add to it, or take away from it. I like what Slime Online did, which is to use preset hotkey emoticons. Stuff like the traditional smilies, and other stuff like "Help" or "Follow me" or whatever else.
At any rate, I hope you take this into consideration. Multiplayer would add quite an edge to your game!
EDIT: As for buildings, I really like the Chance/Fortune house suggestion. Gambling for the chance to win items at the expense of loss of gold or hearts would be entertaining.