Just watched the Rate that Character for Geno.
Sakurai (paraphrased): "The Mii Gunner plays similarly to how I envisioned Geno to play."
With that in mind, we
could, in theory, get Geno as a "Special Mii Fighter" (or a Mii Fighter Echo Fighter" XD), where he is a
specific build of Mii Gunner (set height, weight and special moves), has a unique voice set and the Announcer refers to him as "Geno". Maybe he also has a Final Smash unique to him to set him apart from other Mii Gunners.
While it's not
perfect, it does kill three birds with one stone.
1) Geno becomes Playable, albeit in a roundabout way.
2) A version of Mii Gunner is playable online (same with other character builds and Mii Fighters).
3) Development time is saved for other characters.
Granted, I don't put too much stock in this idea (I wouldn't want
my characters to just be Mii Fighter builds, no matter how special XD), but it is something we should be prepared for.