Fanatical like a Demon
I could think of a number of things, but I'll let you choose.
1) A 16-bit version of an enemy introduced in a modern Mario game.
2) One or more of the platform or other non-enemy sprites from New Super Mario Bros., such as the rotating red shapes, tilting mushrooms, swinging poles (and ropes and vines), water and sand spouts, rotating blocks, floating water bubbles, seesaw platforms, and the bone coaster.
3) One or more of the Yoshi's Island sprites that haven't been made, such as the Stilt Guys, Bandits, Baseball Boys, pushable pots, weight-activated rotating platforms, and the yellow and red Bullet Bill shooters.
4) A better line-guided sprite base, one that actually uses a sensible routine for setting speed. SMW's one is very inflexible, not to mention abstruse.
5) A platform that can tilt at any angle, or at least a specified one.
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I'm working on a hack! Check it out here. Progress: 64/95 levels.