Suggestion 1:

If you have 2 different characters that must meet, let's say a blue one and a red one.  Then place some objects that they must collect.  The red character must collect all the red objects and avoid all the blue objects. and the blue character must collect all the blue objects and avoid all the red objects.  Once all the objects are collected, then they can meet.  If a red character walks over a blue object then they lose and must restart the level - same if a blue character walks over a red object.  

Suggestion 2:

Add in more characters but they all still move together.  You have 2 blue characters that must meet and 2 yellow characters that must meet.  You win only when both teams are together.  

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?