The thing is, Sakurai is going to include a retro character. He already said as much. That designates at least one spot on the roster for a new retro character. Out of the remaining retro characters, Sakurai also said this character would be capable of having a series revival, like Kid Icarus. Most retro characters, like Bubbles, Excitebiker, Urban Champion, even Balloon Fighter are much more arcade-styled games than plot-based or adventure-ish games, and give very little to work with, expansion-wise. Really the only two notable series left that could theoretically fleshed out are Nazo no Murasamejo and Mach Rider (and StarTropics, but that isn't going to happen, so I don't count it). Those series also have two characters among the few retro characters left not rejected by Sakurai at one point, which does give them an edge over other retros.
Now, from those two, Takamaru has a noticeable amount of popularity (for a retro) - which apparently was a factor in Pit's inclusion, and has made several reappearances as of late, including a major one with Nintendo Land, as his series was put on par with series such as Mario, Zelda, Metroid, etc. Sure, it might not be that significant for another series to have this distinction, most times cameos or minor appearances (like Takamaru's other recent appearances) don't really amount to much, but as retros have very little to go on anyway, things like this are a big deal for them.
Also, it's not just that Takamaru has positive factors for his inclusion, it's also that his retro competition really doesn't have that much. Mach Rider's series could be expanded upon, but he really doesn't have too much popularity, and hasn't received too much attention from Nintendo lately. However, he is probably the second most likely retro character, but really the chances of him making it in over Takamaru are fairly slim. Balloon Fighter is the other retro who gets brought up occasionally, and his games have been highlighted recently by Nintendo, but his series is fairly static and not open to too much expansion, and he was previously turned down by Sakurai. Ofc Sakurai could change his opinion, it's been over a decade, but as there is no evidence of such, BF still caries that negative stigma.
Considering Sakurai also said Takamaru could very well show up in Smash in the future, a statement more positive than most retros get, it stands to reason that due to a mix of positive factors in Takamaru's favour, and really nothing that benefits his competition to any great extent, he's the obvious most likely retro choice. Is he the only choice? No, there is a chance another character could get in over him, but considering who's in the running, and considering Sakurai, I wouldn't say it's too likely.
(Dog can direct the hunter to shoot players!