Well in that case, I'll start off with two cheats lol. Fire breath, what his "father" has, and the psuedo telekinesis that Inko has. Amplified by OFA would make those two some solid quirks for all around uses.
As for my other three:
- Super regeneration. Quite obvious. Deku having super regen would be amazingly helpful to help cancel out all the recoil he gets from using higher OFA percents.
- Kind of a double on super regen, but cell activation amped by OFA in addition to super regen could allow Deku to do regen and body control shit as if he was Mister Sinister (one of my favorite comic villains btw). We're talking being completely fine from being nearly horizontally cut in half, easy shapeshifting on the fly, etc. Its like hardening, regen, and shapeshifting all in one. Hell, if the OFA amplification is good enough, I could even see Deku pulling some Mr. Fantastic-like stuff.
Decay.
Just kidding on decay. Can't really think of anything I like too much for that slot so I just put whatever came to my mind first. If I come up with a better one later, I'll share.