So anyway, hot take but what if the simplest reason that "The only Mii outfits that came with Hero were Dragon Quest" is that Square Enix used to be two separate companies, namely, Square and Enix respectively.
They're one and the same nowadays, but just consider.
The Enix part of Square made Dragon Quest
The Square part of Enix made Final Fantasy(And, Super Mario RPG for the record)
Honestly, maybe the reason that we haven't seen the Geno costume is really just 'that' simple. I'm sure you could make an argument that "Final Fantasy will only bring Final Fantasy costumes like Hero only brought DQ costumes", but we've yet to see if that's the case.
Speaking of, although a lot of people would like it, I don't expect a Black Mage outfit with Sephiroth. Final Fantasy in Smash seems tied to FF7, and Black Mage, as Nintendo fans know it, wasn't in FF7.
I could see something like:
Barret(Gunner)
Tifa(Brawler)
Aerith(Swordfighter)(lol staff)
Chocobo Hat
Moogle Hat
But not Black Mage.