So, how good are the chances of the Inklings being included in the next Smash Bros game?
Doubtful. We'll probably need to see a Splatoon 2 before they even consider including them in the next Smash Bros game. (And that of course assumes Splatoon 2 is a success. If it isn't... well. So much for that idea.)
There are inkling costumes for the Mii fighters coming soon, so I doubt they'd be in the current game, but if Splatoon does well enough for a sequel I wouldn't count them out of the next game.