I finally seem to have settled on a name for this thing. I think it sounds vaguely like an 80s Shoot-em-up might sound, and nobody else appears to have used it.

I've spent today working on the Enemy Types, trying lots of different movement styles and things. So far, so good. I think!
Tomorrow I'll be working on different weapon types, and trying not to make the weapons TOO powerful.. You want to have fun with the blasts, but not TOO much that the game becomes too simple to play.

And then it's a case of balancing everything out, and trying to make it fun for a good number of levels, so you don't get bored of the constant trek to find the three disks.
Hopefully I can manage it.

More info to come!