I have one in mind that I already designed and verified last year, but I wouldn't mind hearing suggestions to generate ideas I might not consider otherwise!
Caveat:
It needs to fit in a 1590A or B.
Not too complicated - no analog delays or choruses, etc. It needs to be something straightforward and easy to build with no esoteric parts.
I'd like to do the next Giveaway in March or April so if we start now there's plenty of time.