Pikachu & Jigglypuff are the staples & aren't going anywhere
Mewtwo will most likely return due he is the most requested Pokemon rep & in the top 5 of overall Smash bros 4 characters people want to see
Lucario is underestimated by the Smash bros fans, they think he even is not important to the Pokemon franchise anymore & flavor of the month, but he will most likely return due he kinda is the Mascot of the newer games & has a playstyle that differs himself among the rest of the cast that helps him to.
PT is the most likely cut, due Sakurai said he is not looking forward to expand the Smash bros 4 roster & unlike most people do, Sakurai count his 3 pokemon as 3 seperate characters and If we really are going to get the same amount that Brawl had, his cut would mean room for 3 Newcomers
The PT concept is also very labor heavy to recreate & Sakurai also said he had troubles with characters that have multiple forms on the 3DS version
This, this right here, except for maybe PT since he's important to the franchise and I don't see him going without Sakurai trying to get him in.
Pikachu is the mascot, Jigglypuff has seniority (though there is a chance she can be cut since she was low priority before, I doubt she will), Mewtwo is THE most wanted character for Pokémon and Lucario's very underestimated because everyone throws the "Flavor of the month" complaint at him when he's proven time and time again he's not.
As for the other Pokémon reps, none stand out. Genesect had his thunder partially stolen by Mewtwo and is kind of in a Deoxys position, and Zoroark did not catch on. Sylveon is admittedly arguable (it's hard to say whether it's really the posterboy of 6th gen vs. Mewtwo), but it also has the problem of being leashed to the rest of the Eeveelutions and I can't see one being represented over the rest.