Of the characters who made their debut in Brawl and are considered in danger of being removed (ROB, Ike, Wolf, Lucas, Pkmn. Trainer, Lucario, Toon Link, Snake, Sonic)...
1)I honestly don't see Ike or ROB being removed. They are both unique characters. Sure, Mewtwo was removed from Brawl even though he was unique, but then again, part of his moveset is infused with Lucario. For Ike, I see no way in which Chrom can take some of Ike's moves and incorporate it into his own. For that reason, Ike has a lot going for him, and the only thing against him is that he is not recent enough, and that doesn't mean anything. He is the Western FE icon, and the second most important character in the series behind only Marth.
2)As for Wolf and Lucas, I feel like they fit in perfectly to rep their respective series. Starfox is big enough to warrant three reps, and in no way do I see Krystal more important than Wolf. Wolf represents the villain. And as for Lucas, why go from 2 reps to 1 rep? Lucas and Ness together represent the Mother franchise better than any other in Smash Bros, and cutting Lucas is essentially cutting 1/2 of Mother from Smash.
3) Pkmn. Trainer is arguably the most complex character to ever exist in Smash. His rotation-based mechanics between Squirtle-Ivysaur-Charizard was a dream come true in Brawl. Of all trainers in Smash, Red is most iconic. And Charizard, to say the least, is probably (c'mon people, you know it) the second most popular Pokemon behind Pikachu himself. The only way I see PT trainer being cut is if his mechanics are deemed to complex for 3DS to handle, and even if he is too complex, he could be tweaked.
4)Lucario and Toon Link seem to be in the most danger, with the latter being in even more danger. The only way I see Lucario returning is that he is a mascot for Pokemon. Sure, not on the level of Pikachu, but as people on this thread have stated before, more on the level of Charizard and Jiggs. Couple that with the fact that he is more relevant than Zoroark. He also represents Pokemon's latter gens. Arguably, he could be considered the most important pokemon since gen 1. So, if Pokemon is to get 5 reps, then I could see Mewtwo and Lucario coexisting. As for Toon Link, yes the Spirit Train is a factor to consider, but if Zelda is to have two Links again, then for the second Link, Toon Link seems to have much better chances than Young Link or Retro (Link Between Worlds) Link, because the Toon series is essentially 1/2 of the entire Zelda. So Toon Link's inclusion basically depends on whether Sakurai decides to again have two Links.
5)Finally, third party. Unless Sonic and Snake were meant to be Brawl exclusives, I for sure see Sonic returning. Of all third party characters in existence, Sonic fits best with the Nintendo universe, moreso than Megaman. Couple that with the fact that Nintendo and Sega have entered an exclusive contract for three games, and Sonic's overwhelming fan demand, and I say he has very good chances. Snake, on the other hand, was a strange inclusion, and he was only added in Brawl because of Kojima's begging. MGS 5 is also not coming to Wii U. He also really does not fit in with the Nintendo universe as well as his counterparts, Sonic and Megaman, and in recent years, his presence on Nintendo home consoles has been nonexistent. The only thing going for him is that he is a unique character, and adds a different flare to the Smash franchise.
To conclude, ROB, Ike, Wolf, Lucas, and Sonic have very good chances of returning. Pokemon Trainer depends on 3DS hardware limitations, but I say he also has good chances of making a comeback. Lucario, Toon Link, and Snake have weaker chances, but I do think that Sakurai is trying his best to maintain the Brawl roster. The only way I see these characters getting axed is if Sakurai ABSOLUTELY HAS to cut characters due to time constraints.