I started looking at SMB2.
Interesting stuff:
Animation (dangling fruits, tufts of grass, flying vulture, POW, potion etc) is done by changing tile tables. The ending scene is about 8 full tile tables of
Mario sleeping.
There are a few one bitplane sprites overlaying each other.
The eye-whites are a separate sprite.
Massive amounts of unused tile table space. Maybe because it was a conversion of Doki Doki Panic and stuff was erased, or they just felt they could afford the space.
Some of the sprites seem to have been rushed together. Look at shrunken forms, especially Toad. (Screenshot from my NES tile arrangement tool, the tiles are not ordered like this in the actual game data, but as far as I can tell my tile order corresponds to that of the game.)
Looks like they squandered the space for two complete head frames for running. The vulture probably isn't used, and I think the noise is the potion puff which could be optimized in size (if even used).
Anyways, fiddled some on my own versions. Haven't really tested the 2 frame animation yet. Tried to keep the legs a little ambiguous in Z-order.
Also some SMB1 fun. 4 colors is too tight. 5-6 is enough for almost anything, retro-look-wise.