My thoughts:
Lucina, Greninja (unless it becomes really, really popular like Lucario), Dark Pit (I don't think Sakur-bias will save him even if Sakurai stays on board for Smash), Megaman (because Capcom is practically going out of its way to make sure the character dies), and possibly Dr. Mario (it would be kind of funny if Dr. Mario appeared in every second installment).
I can see Greninja becoming really popular but not Charizard, Mewtwo, Lucario, or Pikachu popular. Just popular enough that Greninja might stay, especially if Gen VII horribly bombs for some reason.
I think Lucina could have stayed if they gave her a damned, unique move set. Same with Dark Pit, because maybe his move set would have been his saving grace, but then we look at Wolf, Mewtwo, and more... Dark Pit should have popped in SSB5 or as DLC while Medusa as a semi-clone of Palutena would have gotten less flak since she's part of the original Kid Icarus and not "Uprising-Sakurai bias". He and Lucina deserved better. Micaiah as a Zelda semi-clone would have been a better idea than Lucina as a clone or alt. Wasted Uprising weapons and wasted chance to just give Lucina Marth's move set and give Marth a fencer move set. Now they'll go down as clones rather than unique characters... I guess, Roy's there to comfort them.
For me, I don't know if Robin will stay in spite of Robin's popularity. Why? Unlike other characters, Robin's not truly defined compared to the Villager, a representation of the player, or Link, an established classic hero. I don't mean Robin lacks depth, I mean Robin's more varied than the Villager who just has skin color, hair, gender, and facial variation. Robin has height, voice, and even slightly different morals depending on the player. Not to mention different classes, but everyone in Awakening had access to class changes. I just have this feeling of uncertainty with Robin. I'd like Robin to stay, but something tells me Robin might not.
Dr. Mario as a on and off character would be hilarious. Now if only Dr. Luigi would pop up. It'd make sense since both of them can't show up or else Mushroom Kingdom will die from disease and injury. Hmm, I guess that explains why Dr. Mario never showed up in Brawl since Luigi's confirmation as a legitimate doctor. Mario was probably educating Luigi on medical practice so he can return for SSB4 and leave Luigi doing everything... As usual, scumbag Mario. As usual.
Capcom is really stupid... People want Mega Man. Gives them a crippled Lost Planet 3. Seriously, the first game was good, the second had a unique multiplayer idea, but 3 became a straight-up copy and paste of third-person shooters. Come on Capcom, we know, Nintendo knows, even Platinum Games knows you can do better and they left to show that Capcom employees are much, much more capable than whatever the hell's going on.
Anyway, I can see Pac-Man not returning if business issues are involved. Shulk might not return considering if the main character of X ends up being way too similar like what happened with Mother/EarthBound's Ninten, Ness, and Lucas; and most Fire Emblem Lords. So, Shulk might just be replaced by the MC from X, have tweaked properties, hit-boxes, etc. Hopefully that won't happen since Shulk has an interesting play-style. Monolith Soft, we're counting on you.
Also, I see Duck Hunt as an underdog-underduck since people freaked out and called BS, but I think the duo are becoming well-liked similar to Marth and Roy in Melee.
Embarrass
)...I mean SmashBoards. lol Just kidding.
