Okay, so he would appear next to the likes of R.O.B. and Mr. Game & Watch?
R.O.B. yes, Game & Watch probably not.
Look at Brawl's CSS:
Pit, Ice Climbers, and R.O.B. are all together because of being Famicom stars. (And Samus is with them because of that as well, even though she's not "retro".)
Mr. Game & Watch is not with them because he is before that time.
So, if a similar thing were to happen, Takamaru would be close to the Ice Climbers and R.O.B. more than he would with Mr. Game & Watch, who is unpredictable.
I would also imagine that like Samus and Pit are near each other here, that Takamaru would also be close to the Zelda characters for his game being a "sister game" (built on the same engine) to the original Zelda like Kid Icarus was to Metroid.
Wii Fit Trainer isn't obscure, just incredibly out there. There's a difference. To give an extreme comparison, it'd be like if they added the mushroom from Mario as a character. The mushroom isn't obscure, it's just the idea of it being a character is so ridiculous that nobody considered it.
........am I being trolled here?