After all geek-hard suggestions, here's a simpler one: How about making the already existing Turbo mode even harder? The thing is, I enjoy replaying old SP levels in Turbo mode but eventually came to notice that most levels are fond of PU's. The result is that most enemies will die in one hit anyway, making the Turbo difficulty barely noticable as long as you don't take more than 2 hits..

So to make Turbo even more fun:
1. Maybe do a x3 or x4 health multiplication on all enemies? I want to mash buttons JJ1 style!
2. Would it be doable to make all enemies move faster, also JJ1 style? A bit like the Schwartzenguard's chain sickle speed already does.