A survival gate to the south or w/e of Haven that allows players to fight waves of monsters that has resting places every 10 floors.
At level 29 or so, there might be a re-colored version of the Jelly King that uses shock.
At level 59 or so, there might be a re-colored version of Vanaduke that uses freeze instead.
At level 89 or so, there might be King Tinkenzar or whatever the Gremlins' king is.
At level 99, there might be a Lichen boss.
A scoreboard is also a must!
Love the idea