I think I'm with
@SiG, in that the greatest appeal of a SMRPG2 for me would be a second chance to explore the "Mario meets Final Fantasy" premise that only SMRPG ever had going for it, for what I'd assume are some pretty obvious reasons.
Not a dig at the other successor RPG Mario series. They just fundamental can't be that, even when sharing some staff, and have had to do their own things(which usually meant playing up the silly times wackiness factor), for better or for worse depending on who you ask and when you ask it.
But a SMRPG where you get a Blue Mage type party member? Or a Bard/Dancer type? Or a Thief? Heck yeah that would be dope!
Really elaborate summons animations; Fakeout ending where you realize you are actually only at the halfway point of the story; The Mario gang gets an airship(!); optional card games you can spend most your time playing instead...
There's really a lot they still haven't tapped into with Mario x FF, without even touching cameos from more recent FF and Mario games, which obviously they'd do that too. But I know that at least for me, simply another Mario role playing game isn't what I'd be interested in just on it's own.
But SMRPG2? That's a whole other beast entirely. Potentially anyway.
A sequel so long after the fact... there's a lot of fan imagination and hype to live up to there. I wish them the best if they actually get the green light.