Not sure if its true (very likely is!), but someone said you can't complete the main game and then jump into the ID and level up past 50.. Someone said maybe it'd mess up high scores, but I kinda always thought that was what ID was for, the last place you go when you've done everything possible in campaign/story mode..
Next I thought of multiple exits to a level that effect the next floors difficulty or mobs (Bloodstained Stairs would be a tougher-than-normal floor if you took those, Slimy Stairs populates the next floor with slimes/molds/jellies) while still offering the normal down stairs, of course.. Don't take the Luminous Stairs
Lastly, mildly off-topic, what about the ability to link ourselves in chat as a creature??
Still keeping my eye out!