A fun use of modern HTML and collecting: Sticker book!
We can get stickers for certain actions, like normal, some need made, some come with achievements... collect and trade with friends! Finishing sets of stickers yields some experience. :]
Sell different kinds of sticker books that change how it looks when viewed. I assume they could be viewed and managed from the website versus in game.
For some ideas, check out www.star.me (http://www.star.me/liftedpixel for my profile).