I really hate to be a pessimist, but I just don't see Ninten happening at this point, and I'm not writing this here with the goal of being a buzzkill, but to rather generate a thought-provoking discussion. If you look at the DLC characters from the previous game, it is clear that Nintendo had faith that each of them would sell either due to fan demand, iconicness, or relevancy.
- Mewtwo - Back due to fan demand
- Lucas - Back due to fan demand
- Roy - Back due to fan demand
- Ryu - An iconic video game character
- Cloud - An iconic video game character that nobody ever expected to join Smash
- Corrin - More of an advertisement for Fire Emblem: Fates
- Bayonetta - Generally relevant and a prominent/promising franchise on Nintendo consoles (supposedly won the ballot)
I don't feel as though Ninten really fits any of these criteria. When it comes to fan demand, I'd say Porky has the most out of all the Mother characters. Forgive me if this sounds rude (because it isn't my intent), but this thread clearly reflects that because it is kept alive by the same 2-3 people. Sure, there could be other places where people are requesting Ninten but I doubt there are large communities dedicated to him, because if there was Ninten would be more of a topic of discussion during Smash speculation and for me it's rare to see him being brought up anywhere outside of Mother-dedicated channels.
Ninten isn't an iconic video game character either. Let's be honest, nobody from Mother is an iconic video game character in the slightest. Some may be known thanks to Smash Bros., but in the larger gaming world Mother is just one of those lesser-known JRPG franchises that people know about but few ever play. Not to mention that Ninten's debut and only appearance is in Mother 1, which compared to EarthBound and Mother 3 is the least popular pick.
Besides getting in due to fan demand or pure iconicness, some of the DLC characters got in likely due to their relevancy, which Ninten and other Mother characters clearly lack. One could argue that Ninten might be added to advertise some special edition of the Mother series thanks to the 30th anniversary, but that's under the assumption that Nintendo and Shigesato Itoi would do something new for the series. Stuff like HD remakes, or any sort of remake, is just highly unlikely in my opinion. Itoi owns the rights to the series and he's made it clear that he's done working on it. If anything, I could see Mother 3 finally being localized, and man that would be weird if Ninten got in alongside a Mother 3 localization (actually, it wouldn't make a lot of sense).
I'd like to end this post with some closing thoughts. At this point, Nintendo would only do something Mother related for the hardcore Mother fans that crave any sort of new content. However, Nintendo has also made it pretty clear that they are not going to go out of their way and jump through hoops for the series. It took them 17 years to finally release the localization of Mother 1, and April will mark 13 years of waiting for Mother 3. I mean once Mother 3 came out, not a lot happened for the series. Because of this, I doubt a new Mother character is in the works for DLC. Irrelevant/obscure/retro characters (which Ninten undoubtedly fits) are best suited for the base roster because they are packaged with the game at no additional cost, whereas for DLC a person would have to go out of their way to purchase the character. It seems that for DLC, Nintendo likes the reach for the stars and pull in characters from all across the gaming world, and compared to the larger gaming world, I just don't think Ninten of all people would be considered a priority in any way.