Aww, that is what I REALLY wanted out of the shop mod, too. In phpBB they have a mod where you can have a pet, feed it, clean it, water it, take it to the vet, and even have it evolve after 30 days. I REALLY like that modification, and it would be fantastic to have it in SMF. I was going to switch the whole forum to phpBB, but then I found out it had security issues where people could get in an mess up your whole forum. So I decided to stick with SMF, but unfortunately it doesn't have the mods I really want.
http://hajinc.com/forum/index.php has an example of the pet system they have.
Another thing I ran across in some forum or another was being able to set up the arcade so that people had to have so many points (or whatever the cash system is) to play a game unless they were a paying member or a staff member. Is there any way of doing something like that?