Trojan(NES colors, enemies, & level layout, with arcade gfx) would rock my sox, as would Faxanadu. Contra & Super C would be great. Karnov, though as I've stated before, I'd have some trouble doing tunes for it, I'd have to maybe borrow tunes from Karnov's Revenge. I suppose I could do just the few tunes that are in the original, & leave it as is, but I feel like it should have at least a few more. Maybe do original tune for level 1, alternate tune level 2, original tune level 3, different alternate tune level 4, etc.
Jackal is of coarse on the top of my list, & I still plan on doing Telenet'ish(think Final Zone II,Psychic Storm, Avenger) tunes for it once I'mdone with Mega Man, though I think Bonknuts has no plans for gfx upgrades. I'm not sure how hard it'd be to convert the arcade gfx to it?