What's interesting to consider about retro characters is that they don't just seem to be arbitrarily added for the sake of having retro characters. In a way, each of them basically represent a piece of old Nintendo hardware.
Obviously, Mr. Game & Watch represents the Game & Watch, Nintendo's first fooray into handhelds. As we all know, the ICs got into Melee because Sakurai wanted to include a character that would represent the NES, or Famicom if you will, console. They were on a list alongside other various one-off NES characters, and Sakurai felt they were the best option. Nintendo's home console business had its roots in the one-off, arcade styled games of the NES such as Ice Climber, so it makes sense why they were added. I see them as representing the NES as a whole, not just Ice Climber. That's my problem with including other NES only characters like Mach Rider and Balloon Fighter, because their inclusion would ultimately make the ICs seem less important.
Now, at first glance, R.O.B and Pit seem like characters that were added to represent the NES console as well, but I don't see it that way. R.O.B., for example, mostly represents the peripheral that ended up saving the video game industry in America, rather than one-off NES games like ICs. As for Pit, something interesting to consider is that Kid Icarus wasn't actually released for the NES when it first came out in Japan (Only America and Europe got it in that format). Rather, KI was exclusive to the Famicom Disk System, the disk-based Famicom peripheral that was the birth place for some of Nintendo's key franchises (Metroid, Legend of Zelda). Argurably, Pit represents (or represented) that more than he does the NES/Famicom itself.
I believe that's where Takamaru comes in. Much like KI, Nazo no Murasame Jō wasn't released on the NES/Famicom, but for the Disk System. With Kid Icarus Uprising, Pit no longer really counts as a retro character, like Little Mac. So, Takamaru could easily come in and take his spot as the FDS representative.
Another interesting thing is that Takamaru is basically to Link as Pit was to Samus. I believe Nazo no Murasame Jō ran on the same engine as Legend of Zelda, like how Kid Icarus ran on the same engine as Metroid. It'd be interesting to see all four franchises reunited in Smash.
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