Wow Frizzil you have some great idea's for this mod and I am excited to see where this goes. It seems like an quick fix to the problem of starting zones getting cleared out on big servers is to make mob spawners never actually destroyable but just temporary disabled. So you clear a room then 'break' the spawners and keep on going. An hour or two later after you have left the dungeon the spawners and chest reset and the dungeon can be run by a new group.
I like the multidimensional idea and I want to know if you have thought about adding a safe area starting town. My idea would be a town of portals that is 100% safe zone inside of a bubble. Give a short roguelike story intro (something evil happened to the world and the town mages created a bubble around our town to protect us, you must travel though the portals and gather items for our survival and figure out a way to make the world safe again) and give all players a hearthstone/town portal type spell. On a new server only a portal to a level 1 zone will be accessible but as more players join and more dungeons are conquered open new portals to higher level zones. You could put a 'mini-boss' spawner at the top of every tower and the end of every dungeon. Killing the miniboss would drop a 'miniboss' token and turning in enough tokens could be used by your party to open a temporary portal to a instance encounter with a final boss.
I have read people complaining about how hostile world is with day creeps and and rolfcondas blowing up your buildings and I think the portal town would solve a lot of problems . Multiple portals would making spawn camping less of a problem. If someone is spawn camping outside this level 1 portal then just take the other level 1 portal to a different starting zone. To make the town safe you could make it so no weapons or items could be used but crafting and using your inventory can still be done.
