How about this:

The game would support something called a sprite pack. This is a custom set of sprites that could be distributed as a seperate file (we would provide an editor). Each world can include whatever sprite packs it wants, and use those sprites for objects in its levels.

For example, with JJ2, there are several polls, platforms, and signs included as sprites (via events) in the game, but they don't work for all tilesets. With Mechaius, level designers would be able to create their own sprites and include them into their levels. The benefit of using sprites over tiles would be the ability to treat them as objects, applying physics, collecting, using, etc.

-Nag