One for all with other users
So I've been watching season 3 when they were in the camp or whatever its name, and when I saw Hagakure training with Shouji it came to my mind: what if she has one for all quirk with the invisible quirk, how would she train it?
I mean, at that time, she was trying to be as quiet as she can and smooth so Shouji doesn't catch her, but this isn't exactly related to the quirk as much as it's related to her abilities, as anyone can train themself to be quiet and smooth while walking to not be heard at least.
Also for other people in the story, eraserhead for example. His quirk is literally to keep his eyes open as much as he can, but if he gets one for all, how much exactly would he train it more? And isn't one for all just increases the strength of the quirk itself (if there is not a quirk then the body at all) that means that it wouldn't increase the strength of body as muscles, because there is already a quirk it is going to make stronger?
There are more characters I thought of with the same thing, Mirio (his training was not for his quirk, it was his body and brain adapting to the quirk) , Koda, Toga (as her disguise depends on the amount of blood she drank) Ojiro, Star and Stripe (there are rules, if not broken then she can control or order anything, the training would be more on the mind not the quirk itself) , Momo (the quirk is a thing, and she trying to focus harder to create two things or more in one time would also be mind training)
Trying to say that short, one for all probably will be useless for quirks that are not leveling, but the user themself is leveling to control it better.