Created a new project in SmileBASIC last night, and started fiddling about with a swirly background effect.

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You can See the effect here. (Kinda, with the usual Twitter disclaimer of horrific encoding quality!)

It turned out fairly well, I reckon, and doesn't seem to be too vomit-inducing after too long.

Methodology

Frame One : Rectangles of "screen height" are drawn across the screen.
All Additional Frames : Smaller rectangles are drawn across the bottom of the screen, then the whole screen is GCopy'd up by a number of pixels, in order to scroll the whole thing up, gradually.
The colours are Sin(n)+Cos(n)*1.25, where n is a combination of Frame Count and X offset.

On top of that, some simple diamond sprites, with various amounts of alpha and darkness.

Now I just need a game to go with it.

Gameplay

I'm currently thinking a nice QTE-Event sort of thing, mixing simple QTE's with the gameplay of Orc Attack.
Enemies move up the screen, and your character does a QTE in order to throw things.
I'm not 100% sure that that'll be fast enough, though. Might need a ton of balancing to get it working in a fun way.