As the second "2D forever" guy here, my thoughts are still GH/EH, but ONLY for adventure... it'd be kinda fun to run through them, as, say, Pit, for shits and giggles...
What would I expect? Either A) one of the Boss Areas from the 2D games (GH boss, AR boss... something like that... Casino Night would be WICKED fun, if you opened both sides and put platforms out there) or B) a chunk of vertical Scrolling level... just for shits and giggles
Too bad this game isn't open source...