[@pkken]I'm not sure what category the Quirk I'm thinking of would fall into, but here's the gist:

The character is born with no arms, but has a metallic "base" where his shoulders should be--think something like the part of Edward Elric's automail that covers part of his chest--on both sides. When he reaches the age where Quirks start to manifest, he begins to create mechanical arms that attach to those bases. By developing his skill and control over the power, he'd be able to alter the arms. They would always be mechanical in nature, but the other qualities he could give them would depend on how far-reaching, powerful, etc the Quirk could be without getting too overpowered.

When I originally thought of this, it was more of a "mechanical" power than a "creation" one, and thus the alterations to the arms consisted of adding things like blades, weapons, grappling hooks, and so forth. But I don't think I'll end up going with that unless some of these other suggestions get denied, because as time went on he'd either have an answer for every situation like Batman's utility belt, or else his abilities would focus on pure firepower. Heroes in MHA tend to avoid lethal force and collateral damage whenever possible, so a hero bristling with guns and swords might not work.

In lieu of making it a more "creation" based power, I came up with the idea that he simply alters the number of arms and their structural properties: Four arms instead of two, arms that are much longer for reach, arms that are big enough to serve as shields or bulky enough to give more strength. But I haven't quite figured out the specifics of this "creation," (the only other Quirk that seems to come close is Momo Yaoyorozu's) and thus don't have a good idea of what its upper limits and weaknesses would have to be yet.

If you've ever heard of the game Asura's Wrath, this character would essentially be an MHA-level version of that game's main character--sprouting cyborg arms, punching stuff a lot, and so forth.

If you have any issues with this, please let me know. I'm also very open to suggestions for how to improve the idea.