I have a theory as to a gameplay reason why Link's corrupted arm has long, sharp fingernails.
Nintendo commonly starts with gameplay first and then works to the story or in-game lore reasons afterwards, especially in Zelda. The arm cover, contraption is likely a way to show Link controlling or channeling his new powers. But why would he need long fingernails? After all, he only gets those nails after the green hand possesses his corrupted right arm and he has normal nails even as it's first having its energy manifest within his hand.
It was only later, when I was watching the BOTW GDC video, where I saw the
art director going over some initial concept art. In it, we see various concept arts shown of "what if" ideas that were never used in the first game that eventually came to fruition in the DLC and some later in the sequel. "Bike Link" came to be in the Champion's Ballad DLC with the Master Cycle, Ganondorf waiting atop a floating island, the broken Master Sword, a corrupted am, an arm that uses powers, etc.
But there's a concept art that I still think could be used in the sequel - what they called "musician Link". The image they showed to represent that idea was tracksuit Link playing a Triforce guitar with a triforce pick. Would Link play an electric guitar in the sequel? I find it unlikely given that the concept was already used in Majora's Mask for the Zora Guitar. So if "musician Link" returns (which I think is likely, given that instruments are frequently featured throughout the series), what instrument could he play?
Well, that's where the "gameplay" reason behind Link's long fingernails on his right hand come from:
he's going to use them to play a stringed instrument. What kind? I've looked over various instruments that are plucked/strummed using nails of some kind and there's two that I found that best fit the "musician Link" concept art and utilize these new longer nails.
A classical or acoustic guitar...
or a traditional Ainu instrument, the Tonkori
What do you all think? Will "musician Link" return? If so, what other potential instruments could he play? Is there a gameplay significance to Link's longer fingernails or is it purely aesthetic?