You know what.
I feel that Geno actually has some chance now if he doesn't come back as a Mii costume. I couldn't see it happening before, but now, with the presence of characters like King K. Rool, Simon and Ridley, I feel like he's basically almost guaranteed, or at the very least, would be heavily considered.
Not holding my breath on it, but, who knows? Anything can happen now!
Although I'll say Isaac is more likely, just solely based on that image Sakurai posted on Twitter that looks like it's supposed to be a Golden Sun battle. That puts him in the "very likely" category for me.
Seriously now, all of the newcomer characters so far, aside Inklings who were expected, took us by storm.