Gonna be honest, people keep talking about new series to add to Smash, but honestly I think Splatoon and Rhythm Heaven are the only two series Nintendo has left that I think warrant an inclusion. Looking at the new franchises Sakurai added to Smash over the years, those two are the only ones that actually fit in line.
The rest are really small time series like Chibi-Robo!, Fossil Fighters, Wonderful 101, etc that are kinda the reason Sakurai went ahead and made trophies as well as assist trophies. I'd say how many new series we'll see added in future Smash games really depends on how many new major IPs Nintendo's internal studios make in the wake of Splatoon, or maybe one of those minor second party developed series like Golden Sun suddenly coming back to life and becoming super popular like Pokemon or something.
I actually agree, although a lot of supporters of more minor characters are going to take issue to my saying that.
I'm willing to extend Golden Sun a bit of courtesy because the first game actually sold enough to make it onto a high sales list and what Isaac's capable of seems VERY interesting, but anything below that I feel is at the very least noticeably unlikely. I'm not saying the characters suck in the slightest, as many of them actually do have interesting potential, I just don't think it's likely they'll be in Smash due to how little of an impact they made in anything OTHER than Smash roster creation.
Now granted, we've had our exceptions, but the last full exception we've had was Shulk, who while obscure himself had the whole Project Rainfall thing behind him with his game at the forefront. Most of these characters. . .don't exactly have an army of loyal fans rallying behind them desperate to get their games brought to their shores.
I could see Shovel Knight getting a trophy, though. Nintendo is actually publishing his game in Japan. I think that's why Rayman got one in Smash 4 (they published Rayman Legends in Japan).
My take on it is as follows.
Trophy? Yes.
Mii Costume? Could easily happen.
Assist Trophy? Who knows?
Playable? . . .Doubt it.