They have a window of 2 years to get the 6 more fighters out (by December 2021 or somewhere around then according to that Byleth showcase). That seems like a long time for only 6 more fighters. I don't think we're in store for more fighters mind you, but I wouldn't be surprised if something else for Smash comes out within the 2 years of development left for the game.
Specifically, I could see a paid expansion of sorts get released for the game. Seems like it'd be a way to make a lot of extra easy money, Smash players will happily buy what more content they can offer for the game, and Nintendo seems to be embracing the idea of expansions more and more these days. I think this idea has more merit when you consider the words Sakurai specifically said about Home Run Contest being the last mode to get added to the game. The way he said it was, it was "pretty much" the last mode that would get added to the game. Perhaps, that qualifier of pretty much was his way of saying, this is the last free mode to get added to the base game, but more modes may be on board via something like a paid expansion or something similar.
It certainly would make sense, there are a lot of missing great modes from Smash's history for a game titled 'Ultimate': Break the Targets, Boss Rush, All-Star (proper), Smash Run, Smash Tour (for the few that enjoyed it), Master Orders, Crazy Orders, Board the Platforms, they could give us HD versions of Melee's Adventure and/or Subspace Emissary (obviously not even holding my breath on those adventure modes returning even if there were no real changes to those modes). And heck I could think of a few more great modes or options they could add like a Boss vs mode where one player is any of the bosses, and all the other players fight together to take that player down, or an option to toggle a fighter swap mode where at the start of a match, you choose 2 fighters that you can swap between with either a taunt, or random or timed intervals. Plus there are still a few classic stages that are omitted that would be welcome by many players.
Obviously if such an expansion were on the cards i wouldn't expect all of that to be added, but it just goes to show that Ultimate has plenty of content that could still yet be added beyond some new fighters and accompanying stages.