As far as uniqueness in smash bros. being your point, Takamaru fights with a sword. We have none of THAT, right? And I know he can do other things, but his main smashes will most likely use his sword if he's playable. Whereas Lil Mac, is a fighting style we haven't seen yet. Sure we have other fist fighters, but we don't have other boxers. However I could see this being used as a point for Takamaru as well, being that we don't yet have a samurai. But when it comes to a japan only smash contender with moveset potential? Sukapon. However Takamaru has a bit of popularity as of late, so I think that's why we mention him so much, and is what sets him apart.
Also, calling Punch-Out!! a retro game at this point is like calling Kid Icarus one. It's obviously not anymore. It used to be....back when it WAS, you know. Just like any other retro series that is still around today. But Little Mac is super important to Nintendo, I've argued with friends about it to star fox actually. there's no doubt in my mind Mac will be playable, but Takamaru is no shoe in if you ask me. Pretty likely, but no shoe in.