I think a lot of people forget that licensing rights are very tedious and specific.
Just because Geno was a costume in a previous iteration, doesn't necessarily mean that SE gave Nintendo permission to make him a character for a different title.
Also, on the topic of SMRPG and their spirits/icons being in the game, it's probably Sakurai and the Smash team requesting for their imagery/likeness.
Not necessarily Square Enix warming up to the "idea" of their previous cross-over and giving it to them. Just the fact that Nintendo was willing to pay them for it, and they agreed to it.
It's kinda the reason why very few FF7 content is in Smash.
It's not that Square Enix hates the franchise and refuses to put any content.
In fact, I'd say it's the opposite. They use Cloud and the other FF7 characters to this day, and they recognize and embrace that it's a beloved and iconic game.
It's just the rights to use the imagery and songs cost too much for Nintendo to even bother with, so it's not placed in the game.
It's a bit of false equivalency.
Also, like many other people have mentioned before, SE focuses on characters that promote a product or/and have some height of popularity.
2B, Noctis, and Cloud all hit those marks. They're practically recognizable to a large swath of people who play games, and those games or associated products still make a lot of revenue for SE.
And at the end of the day, Geno straight up doesn't hit those marks, nor will it bring in a large crowd like the rest of their IPs.
He's relegated to an "underpreformed" spin off game, where it was the only time he was playable or heavily featured. While, yes, the dedicated fanbase for him got a costume because of it. I'm afraid that's all they're going to land.
So based off of all of that, I honestly don't see him happening for DLC. So I definitely wouldn't keep your hopes up.