Ha! I sure do! You can crank it up to seven stars if you beat all the hard challenges in the Mission Matrix. The hardest challenges unlock more lives. Beating them all unlocks... well, you'll just have to finish the Mission Matrix and find out!
I used a variable called dynatime (short for "dynamic timer") to change the timing for each stage, so the simple stages with faster execution speeds would run at roughly the same speeds as, say, Galaxians, which makes greater demands of the GameBoy Advance hardware. Looking back, I'm wondering if I should have adjusted dynatime slightly for the Astro Battles and Flagship stages. There are a couple of other minor bugs in the game that I should attend to, but right now, I'm all GORFed out and need a break.
(I considered Wizard of Wor as a follow-up, but the game was really dependent on two player play, and Dragon BASIC doesn't seem to support system link functions. I'm sure there's a way to replicate those functions with PEEKs and POKEs, but you can't make a second player with them!)