Simplifying a 6-string guitar to 5 buttons is much different from simplifying an 88-key piano to 5 buttons. If my math skills are correct, you'd be getting 17.6 keys-per-button if you simplified it that way.
Who said it would have to be 5 buttons? Doing piano with one hand just seems off to me. Despite the guitar only having 6 strings, holding the string in a different position causes a heightened or lowered pitch in the note, which is why there are notes played with orange and a higher note still played with orange. It's not right as a musician. Playing Guitar Hero or Rock Band with piano would actually be more fun. More challenging yes, but more fun. I say 20 keys to simplify 88 keys would be best.
But like I said in my previous post: Not many songs in either games have tech parts which can be played with piano. It would have to be left out many times more than a mic would for those rare instrumental-only songs.
