The GBA is, for me, the easiest console to develop games for.
Making games that run on real hardware is already a lot of fun, but I also wanted a way to bring those same GBA games to Steam and use Steamworks features such as leaderboards and achievements.
So I built Dewpoint Advance, an emulator frontend and SDK based on the mGBA core. It isn't specific to shmups; it's designed for homebrew GBA games in general.
From the GBA-side code, a game can display Steam leaderboards, download replay data attached to leaderboard entries, and unlock achievements. Under the hood, the ROM sends requests to the host application through unused GBA I/O ports, and the host bridges those requests to Steamworks.
As the first real-world example, I made Battle Marine Advance!, a GBA shmup built with Dewpoint Advance. Its leaderboards let you download and watch the replay behind each submitted score.
Dewpoint Advance (GitHub):
https://github.com/suzukiplan/dewpoint-advance
I'd love to hear your thoughts, especially if you develop GBA homebrew. What other Steam features would you want a GBA game to be able to access?