1) sprite-to-sprite collision detection - It would be very helpful especially in games like bullet hells.
2) Debug mode - Probably won't happen, but it would be SUPER helpful if there was a debug mode, where you can slow down the game and see the code so you know what causes problems.
3) User Profiles - On the website, it would be cool if we could see user profiles and see what games they have made, so if you like one game that person made, you don't have to search through the list.
4) Game searching - It would be a lot easier to find games you want if you could search for games by their name, or sort them by a tag they can choose, or both!
5) A thing - kind of minor, but maybe move the default map position up a few pixels so we can see the top row.