Takamaru joining Smash Switch is clear as day, in my opinion. We have already seen Nintendo and Sakurai dedicating some love to the Famicom game with Nintendo Land's mini-game, 3DS Virtual Console release, Takamaru's AT role and Mii costumes in Smash 4.
I feel what happened back in Smash 4 early planning in terms of the newcomer selection was that one retro and one retro hardware representative were planned from the very start. Duck Hunt Dog was the first selected to be Smash 4's 'surprise' character according to the
Smash 4 slideshow proposal. Takamaru, was not in specific competition of that category. I feel, Takamaru was competing against Little Mac for the only standard retro newcomer spot. Although both had strong reasons to get into Smash, I think Mac was ultimately chosen over because of his worldwide appeal, especially noting the retro revival game that was Punch Out!! Wii that came out in 2009.
Being the
only category Takamaru likely had a chance to be chosen in, it is VERY easy to understand why Takamaru could not get in Smash 4 despite merits. Getting in as DLC was not an option for Takamaru either, as I feel it was likely the retro category was hardly considered for the DLC selection. Now here is the interesting part.
Assuming that development started around in 2016 when Sakurai confirmed that
he is working on the next project (likely being Smash Bros. Switch), there were no notable retro revival games from classic franchises from Nintendo that came during the timespan of Smash 4 and now. Given that selection of newcomers were likely selected from the beginning, Sakurai will have likely chosen the next standard retro candidate the same way as he did in Melee and Brawl (With Pit and Ice Climbers coming to mind). With no retro revivals to compete with, given everything I explained here, with Smash 4 content and pushes of the Murasame game from Nintendo, and one spot likely being reserved for one retro character again like Mac, with everything considered, it all lines up for Takamaru to be the next contender.
Sakurai had already noted the game's historical relationship with Zelda, Metroid, and Kid Icarus as an important point of Takamaru back in the Pic of the Day. I feel that status gives him the edge of consideration also. The only thing that would stand in the away for competition is if there were a major retro revival for a classic franchise other than Murasame Castle that Nintendo wants to promote badly, assuming the game comes out in 2018 in time for promotion. But I feel that is very unlikely at this point in time. I would be very surprised if anyone else got chosen other than Takamaru, unless in the case that Sakurai reconsiders Balloon Fighter and puts him in competition of Takamaru in honor of Iwata. Those are the only two candidates I can see Sakurai reconsidering for top retro candidates again. That and obscure status potentially being very large obstacles of Sakurai adding Takamaru, those are his only problems I can see. But, I can see Sakurai getting past that pretty easily and taking the risk to make the samurai finally playable in Smash.
Takamaru, to me, is literally in the very front of the line to get in Smash now.