With a game like this, there are many opportunities for improvements. Your own imagination is the best source for enhancing this game, but here are a few ideas for making this platform engine more interesting, or just better:
Add more types of enemies.
Create a boss at the end of each level that the hero must find a way to defeat.
Come up with a story line that can get the game player more involved. Give the hero some sort of motive for what he is doing. Why is he collecting things? Why is he killing enemies? Where is he going?
Create more graphics for different types of platforms.
Add angled slope areas that the hero can slide down.
Give the hero some sort of gimmicky behavior, such as flipping, super speed, or a special type of weapon (like shooting fire).
Spice up the visuals with several different types of backgrounds.
Add another level of visual parallax. (For example, add something between the background and the foreground, which scroll at different rates.)
Create more types of collectable items, including rare ones.
Make the code more efficient. Currently the game code has one major disadvantage it is checking for collisions between objects that are not even close to it! You can use the tile-based approach or some other type of "zoning" that will allow you to only have to detect collisions between objects that are in close proximity.
Enhance the level editor so that it can load in levels and then let you edit them that way so that you don't have to build from scratch every time.
Enhance the level editor by including a "test" button or other method for getting to a test state. You can easily write the XML document to a shared object and launch the game file to automatically test the level.