I'd prefer you just get your choice of the "roulette". It's still just one bonus per category per day (call them "bonus" dungeons instead, if you like). This gives incentive to participate in all levels of dungeons, and without the problem of people dropping from instances they dislike. I would split the "low" level into 2 though, about 1-31 low, 31-49 medium, something in that ballpark.

Now, without the "roulette" part, you have a very easy way to stop people leaving. If you leave a "bonus dungeon", no more bonuses for you that day. Disconnections need to log back in within a set time, or take the penalty. If you choose to do a bonus dungeon, you cannot initiate, or vote on a kick.