This first idea is one I've already seen done before, but unfortunately, the fan fiction seems to have stopped updating as of 2019. For this idea, I will be explaining both the idea I'm borrowing, if anyone wants to try their hand at something similar to it, as well as an alternative option you can take.
The idea I'm borrowing basically has it so that Touma's left hand reflects powers back more powerfully than the original strike sent at him. That's all there really is to that idea, so I was thinking it could maybe be expanded upon. We know that dragons lie under Touma's right hand, and that Imagine Breaker may act as a seal for them, but what about his left hand, if you give that a power as well? You could either just have it be another eight dragons, or you could perhaps have different mythological creatures underneath there, such as sea serpents, or something.
As for some alternative powers that you could give his left hand, you could make it an esper ability, if you ignore the fact that Imagine Breaker should make that impossible. Other than that, maybe you could make it so that's it like an "inventory" of sorts, or in other words, it could store attacks that hit it, and they can then be used at a later date. You could either make this a situation where only one attack can be stored at a time, or you could make it so that multiple can be stored, instead.
Perhaps you could even make it so that this "inventory" can store regular, non-esper or magic related weapons, too? It'd be a nice way to get around one of Touma's only weaknesses, plus it'd be interesting to see him just pull a knife out of his left hand in the middle of a fight, even if Touma's more of a fists-only sort of fighter, usually. I'm not really sure how the logic surrounding it would work, however, even in a world like Toaru, and though that is the only real problem I see with it, I'd still recommend trying to make it more realistic than what I've described. Of course, if you don't allow non-magical or esper related attacks to be stored, then that problem mostly sorts itself out.
As for the second idea, I don't really have much to say on it, other than that Imagine Breaker covering Touma's entire body seems like an interesting concept for a story. I can't really say anymore on it, because the idea can pretty much be summed up by just those couple words I've already written. In fact, the entire reason I put these two ideas together is because this idea is too short for me to make a single chapter on it. It would be quite intriguing to see the changes it would bring to canon, however, and I'm not sure how the "whole 'invisible thing' coming out when his arm is chopped off thing" would work. Maybe just have it come out when any one of Touma's limbs are cut off?