Those would be indeed really cool to see in the game!
I've actually been doing some "historical" gaming posts lately, covering noteworthy games released in certain years (just made two about 98 and 2000) and there are actually numerous Nintendo characters that nobody really remembers about, but who would make awesome playable characters.
Also Hive account@rogueofoz has a whole series of posts about lesser known Nintendo characters that is really cool to read!