Hello, and welcome to my theory corner! In this episode (see others on Sonic News Network), I'll tell ya about the Zero/Light theory.
Physical and Canonical Status
Mostly, it's about that almost every robot Doctor Albert W. Wily is or was built by Doctor Thomas Light. Also, Proto Man has a striking resemblance to Zero, implying (depending on this theory is true or not) that maybe Blues would get an upgrade. However, that evil Doc Brow--I mean Doctor Wily stole the parts.
Quick Man Status
Aside from the fact that Zero is a living tank, he's also very fast. This leads me and others to the theory that it was at first, supposed to be a sort of "ProtoQuick Man". The evidence is well, Zero's quick and can face Mega Man X (character) without a sweat, being able to run circles around X, even with the Ultimate Armor on. Also, like his Non-X possible counterpart, he can ram into Mega man and deal massive amounts of damage by just touching him.
Metal Man Alt. StatusA counterpart to the Quick Man theory is that Zero is the X-counterpart to Metal Man. I personally believe this more than the Quick Man. However, there's one more thing to do...
Zero is another version of Mega Man (character)'s Armor
This Alt. Theory suggests that both Alts are right, but Light made it so Mega's two archrivals would be his powers, even them being disguised as something else. (Quick Boomerang = Zero's Sword Blasts, Metal Blade = Sword) I think this is the best answer here, but that's just me, y'know?
Well, that's all the time we have today, please tell me what you think, and leave your want-me-to-cover theories below! Goodbye!