I personally think that something like a dungeon plugin would be most enjoyable. This would make for better adventuring on the server, something that pure vanilla minecraft often lacks. I've already looked at a few mods that add things such as grave yards, sky fortresses and extra strongholds. It makes exploring the land a blast and adds a lot into the game.
Another thing that might be considered is extra bosses. I haven't looked much into this but I like the idea.
So please, if you like the idea of a small add in like this, please share and post a reply!
Thnx
-Karl