Low gravity patch is definitely an example of something not allowed for that reason.
And regarding creativity, I think it's definitely possible to use sprites in clever and interesting ways while also not being an enjoyable experience as a whole. I find the Fishing CLDC entry to be a good example of this, though of course fun varies from person to person as seen in that scores for that level.
I will also add a note about PIXI.
As for SA-1, I don't think there's any problem with that. Just note that not all the sprites may be SA-1 ready.
(Erik will have to clarify sprite specifics since that's not my area of expertise)
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