I'd be for e.g.
a temporary "tilt screen"-mode (or reversed/mirrored keys: left = right, up = down) if pulling a lever or clicking a button
pits with thorns/water/lava
optional checkpoints/marks during levels
coins to collect & use for upgrading/armor/potions/etc.
chests that include either goodies (items) or some encounters (enemies)
lv's with amounts of objects like: "hidden item found"
unlockable secret passages
get onto the other char's head to reach higher spots (teamwork)
stand on portal with one char & get into a door which would be locked in that portal isn't stepped on
walls or floors that narrow/widen in pixels
floor-tiles that's made of ice (sliding-effect to corners as "maze" to its exit)
those are just a few that came in mind, sure I could refer to even more suggestions too?