Warning : this post isn't a programmation post , it is just on how you can pump up your platform game
let's start by the player...
Power-ups:
you can add random spawning power-ups to your game to make the gameplay harder or easier ..
like player transforming into a big monster who destroy everything
or
player have special ability that kills all in-screen bad guys..
or
player transform into a heli, a plane, a car, a bird or anything in your mind
or
player shrinks
or
player speed up and make all enemies slow
or
player disappear
or
everything you like...
Puzzles:
puzzles are always liked ... or hated :D
you can make the player have to get the key to open door
or
respond to a question to open a door
or
beat-all-enemies with a puzzle like order
or
everything you like...
Bad boys
slime who trows other shrinked slimes
or
Dark copy of the P1
or
Big monster
or
etc..
Here ya go
i hope you got ideas from here