What if there was a little more choice when you visit the nailsmith? I think having different nail upgrade options suited to different playstyles could be cool, sort of like choosing a specific class. Nail upgrades would probably cost the same regardless of what you upgrade to.
There could also be a way to reset your decisions, if you decide that your nail is no longer for you (or maybe not, undecided on this one).
I've attached a hopefully balanced upgrade tree (pardon my lack of artistic talent), the idea is to not make any nail significantly stronger or weaker than the regular nail upgrade system, to not mess up the game's difficulty.