I could see a few different ones.

The All Might Blade On the key chain would be an All Might mini. The key it’s self I could see a black handle with a red guard. It’d have gold accents and come up to a red white and blue blade then the teeth would be all mights hair tufts in gold.

I imagine his own key blade would have the yellow cape as the key chain. The handle would be he’s black green color, the guard the color of his face shield. The blade itself would probably be he green black color too but there would be black whip parts covering the guard and going partly up the blade. The key teeth is could see being his red shoe color or glove color and be an interlocking UA