I have no doubt that 3H is a dope game, but even FE fans at this point must be wary of the prospect of ANOTHER FE rep in Smash. It's come to the point where I feel like instead of promoting the brand/new game, a new FE rep would be ACTIVELY ANTAGONIZING the Smash install base (not that I think that would be Sakurai/Nintendo's INTENT, but that would be the end result), and would just end up generating ill will toward the franchise.
I'd be fine with Pauline as an ECHO, or an alt, or something along those lines, but Pauline as a distinct newcomer feels premature. Give her a prominent role in an Odyssey sequel, or throw her in a few Mario spinoffs, then I'd be less against the idea. Though I still think any additional Mario rep (barring Waluigi, who at least has a strong demand behind him) is overkill.
I may be alone with this, but I always imagined Geno as kind of the Silver Surfer of the Mario universe. I could see him being voiced by Michael Rosenbaum, if anyone.