Include pricing and such for DC comic cards. I've got a few sets outside of the Marvel ones, I'm sure others do as well.

add all union arena sets to the site

Add One Piece Japanese TCG 

Make a database of action figures and track their values like you do for games, cards, and comics.

Please add support for PAL PC games or add PEGI and 13 digit UPC support on NTSC PC games. Thanks!

Create a tool for users to create a custom set or list.  Then let users collect against that list and see what they have in their collection and what they don't have yet.

Examples: a list of all licensed NES games, Mega Man home console games, games published by Konami, etc.

It would be great to have a button on each wishlist item to move it to your collection. Right now, the only way to (indirectly) do this is by clicking on the wishlist item, then clicking "See Historic Prices" on the item page, then finally there's a button to add it to your collection. This would make updating your wishlist much more convenient and would also give that tiny endorphin rush from "completing" an item of your wishlist.

This would help a collector quickly track which games on their collection page are increasing or decreasing in value. Would be good to add to a sellers listing as well.