Well it would be the same sprite, just edited to look "right". Samus has an outline, for example, that is not present in Metroid. This would be more Xtreme (pardon the pun), but not unprecedented. Iiiiii...derped entirely on that. You're right about his jump. Although Wall Jump would work in some scenarios, there are many where it would fall short of being any kind of helpful. Maybe the mid-air dash would make up for it? A high jump is also viable, but not at all to the extent of Mega Man's.
I know it's an old topic but i wanted to share my opinion after reading through. IMO the GBA series have excellent gameplay. We could use custom spriting with the GBA moveset. The wall kick can supplement the low jump, as it's used that way on the original games anyway. Sword combos would be good if they weren't so not-8-bit. Zero is much more agile than X because the game itself is harder. Mostly because enemies could come from any direction while on the X series you could only shoot straight. Also i think the new design from the GBA series is better for two reasons: 1.It differentiates him more from the X series and 2.It's less bulky; Zero's moveset implies he's very light.
I think that this GBC game is simply the palette to use, not the game that makes people want to have Zero in this game. For those that have played the GBA games that feature Zero, then it seems like perfect fit for SMBC.
I would for once like to see a version of zero that's not from the x series in a crossover game. I remember there was 8 bit Zero sprites from the zero series somewhere but I wouldn't like to take sprites from someone else,I would like to at least have the x version in the game. /support
Definitely not in addition to Mega Man and Bass. If it's a skin, go for it. As a whole new character? No.
If it was a whole another character, how would you make it available when you turn ACTIVE SKINS to NES ?