All five DLC fighters for Super Smash Bros. Ultimate have already been decided. Series creator Masahiro Sakurai confirmed this in a tweet, saying that Nintendo itself chose the characters.
This is interesting, not only because all the fighters have already been chosen, but that Nintendo itself made the decision of who to include. This is an important distinction. According to Sakurai here, Nintendo, not he, picked the DLC characters. Nintendo is a company, so it wouldn't be wrong to assume that it would choose fighters with the broadest market appeal. Super Smash Bros. Ultimate is one of Nintendo's biggest games ever made, so it makes sense that the company would want to be the decision maker of who to include.
DLC content was confirmed at the recent Smash Bros. Nintendo Direct. So far, all we know is that there are five (six if we count Piranha Plant) DLC fighters, complete with their own stage and music tracks. Now we know that Nintendo decided who to include. It is hard to guess who these characters could be. Shadow the Hedgehog and Isaac being Assist Trophies rules them out. Nintendo has a good relationship with Microsoft, so Banjo-Kazooie is not out of the question. However, one has to also consider that Nintendo would want to cross-promote with current and maybe future game releases. Corrin was included as DLC in Super Smash Bros. for Wii U and 3DS as a preview for Fire Emblem Fates, as an example.
It is fair to assume that a majority of these fighters will be first party Nintendo characters. Of course, the company could surprise fans by delivering more third party fighters, like Shantae (who is a Spirit within the game) or Steve from Minecraft. It will be fascinating to learn who that first new DLC fighter is. No release dates were given in the Direct, but we do know that all of the DLC will release by the end of February, 2020. So, one can expect to see maybe three or even four of the fighters through 2019.
Super Smash Bros. Ultimate releases next month, on December 7th.