Well, Mega Man and Pac Man are always going to have the axe hanging above their heads due to them being third party (Although I can't see why Capcom and Namco wouldn't want them in the next Smash).
Lucina. If either of the new FE characters don't come back it's going to be her that gets cut.
Bowser Jr. Mario has quite a few characters at this point, and if one gets cut it's not going to be Rosalina.
Greninja. I can see him becoming the next Mewtwo.
Unless something severely goes wrong with their relationships, no way any of the three 3rd parties get cut. They are too iconic and fit in too well.
I'd say theres around 75% chance that Lucina stays. She is very popular and she can stay as an easy clone. She is the first true female FE rep and that series needed one considering how equal it is between genders. The only FE cut I see happening is Robin only to be replaced with the next customizable Tactician with probably the near same moveset AKA just updating the Tactician's design.
No way Bowser Jr. or the Koopalings get cut. They are too important to the NSMB series and Mario is their top franchise, it can have lots of characters.
I'd say theres a 90% chance Greninja stays. Very popular and I don't see his popularity declining over time much at all.
And Dark Pit will likely stay because of Sakurai bias
For Smash 5, I think there is a high chance we don't see any cuts. Any character added inSmash 4 seems to have staying power for one reason or another. I know that could possibly bring it to 60-65 characters next time which seems insane but theres no real reason to cut any of the characters next time. The only true cuts for Smash 4 were Lucas and Wolf since IC was because of 3DS and Snake was because of Konami. Sakurai wouldn't have cut either since they are entire franchises unless he had too.
I just don't see any character being cut next go around. Will be interesting to see which characters if any get cut for Smash 5.