Smash 4's thing seemed to expanding the amount of franchises, especially with DLC in consideration
We got Animal Crossing, Wii Fit, Punch Out, Mega Man, Pac- Man, Duck Hunt, Xenoblade, Street Fighter and Final Fantasy, nearly as many franchises as the original.
I think for Smash 5 it will be the expanding of existing series like Melee did to the big three, Brawl did to the others and 4 did for Kid Icarus, Fire Emblem and Mario (again).
We gotta think of what characters are iconic to their own series. Like Dixie Kong and King K. Rool to Donkey Kong; Tom Nook and Isabelle to Animal Crossing; and, if we dare go there, expanding 3rd parties beyond one character.
Sonic, Street Fighter and Final Fantasy are full of playable characters, Mega Man has a lot of supporting characters with a lot of boss weapons to steal, Pac-Man may be a problem unless expanded to include all of Namco's arcade line-up.