To be fair, SaGa 3 (the original) was the only one with real consistent characters of the original three, though yeah SaGa 2 did have the Temp PCs which I suspect they'll be using. SaGa 1/2 were in FF1's style of "Create your team!"
That said, SaGa isn't as big of a series as Final Fantasy in terms of raw scope. It has 9 games total I believe? The 3 originals, 3 Romancings, 2 Frontiers, and one Unlimited, and after that it's been remakes. Japan seems to favor the Romancing Saga games (which go figure are the ones we didn't get here save Minstrel Song) from what I've seen, so makes sense that would be their focus for this, with Frontier games being later add ons.
To be honest, I'm kind of surprised Dragon Quest Record Keeper wasn't their follow up, given Square-enix likes to go "hey, this idea worked, let's do it with Dragon Quest!" Doesn't matter if it's their own franchise or not; I mean they saw Hyrule Warriors and went "Let's do this with Dragon Quest!" for example.