Back in my C64 days, I had a piece of software, by Activision (I think) called the "Video Game Construction Set". It allowed you to create games, either a side-scroller or a top-scroller. You could design sprites, enemy patterns, even sound f/x and simple music. It was a lot of fun and I created two games - a Star Trek based top-shooter (the Enterprise vs. Klingons and Romulans!) and one based on the old "Hunter" television show (hey, it was cool in those days!).
Anyway, what if someone did something similar to that now, where you could use your Gamecube to make a game that you could play on a GBA? Imagine being able to use scaling, paralax scrolling, maybe even mode 7. You could save the game onto a memory card to swap with someone else, and then download the game onto the GBA or play it on your Cube. Perhaps they could even add in a couple of different types of games, say a fighter and RPG (like those Playstation construction sets), but they would be 2D.
I don't know about everyone else, but I remember that old piece of software fondly, and being able to use some of the more "modern" features of the GBA would be very appealing, and a whole lot of fun. What does everyone else think?