That is what I am thinking as well, right now outside of Plant (who was likely initially plan for the base game) we are 2 for 2 in terms of our first party characters being from the Switch era and it would make sense that Nintendo is more incline to pick recent characters from ongoing series over retro characters from dead series.It'll be a Nintendo Switch focused pass if anything.
ARMS and Xenoblade 2 because they're successful, requested and cost nothing to license. A Pokemon. Travis and an SMT rep because they're already being bankrolled by Nintendo on Switch a la Bayonetta so they have vested interests in their success. Last slot... I dunno, a cheap-to-license character from a partner like Terry was (Ryza and Koei Tecmo?).
Though the main thing against this is that Nintendo and Sakurai made several logic games that would have perfectly fit this theory like Astral Chain and Ring Fit Adventure, spirit events.
I also just realize that while Xenoblade 1's party are Spirits in the game, there wasn't a Xenoblade Spirit event despite there being characters that would make sense as Spirits like Riki's kids from FC, the party from Torna The Golden Country and any of the series villains outside of Metal Face and