Lucario is the most likely I would say, but I would put either Toon Link or Ike as the second-most likely character to be cut.
Overall, here are the characters I think have a probability of being cut and the brief reasoning for each of them:
Lucario:
Primarily placed in to represent Diamond/Pearl, his time in the spotlight is now gone, and most of his popularity with it. Will likely be replaced with a new "flavor of the month" Pokemon in Smash 4.
Ike:
Primarily placed in to represent the Tellius arc in Fire Emblem (FE9/FE10). FE9/FE10 were among the poorest selling games in the series in Japan. The Tellius arc will likely not be revisited for this reason. If cut, Ike will likely be replaced with the most recent Fire Emblem lord at the time of development.
Toon Link:
Primarily placed in because he was the "newer" Young Link and was still receiving games at the time of Brawl's development. Since Toon Link will be going into "retirement" according to Auonoma, and Young Link's games are being remade, its quite possible Toon Link will be swapped for Young Link, or an entirely new Zelda character will be placed in.
Wolf:
Krystal's fans are whining that she deserves a spot. However, Star Fox certainly does not deserve four slots. There is a good chance that if Krystal does get in, it will involve Wolf getting axed.
Snake:
Primarily placed in due to a personal friendship between Sakurai and Metal Gear creator Hideo Kojima. If Sakurai does not return to direct, there is a good chance that Snake will be axed seeing as his inclusion was basically a personal favor. Also, we do not know Sakurai's or the Smash Bros. series' policy regarding "guests". In most fighting series, "guests" are dropped in the next game and usually never return.
Sonic:
See the latter explanation for Snake. We do not know Sakurai's or the Smash Bros. series' policy regarding "guests". In most fighting series, "guests" are dropped in the next game and usually never return.
Squirtle:
Pokemon Trainer may be dropped since so many characters are needed to be programmed for his slot. Also, there is the possibility that Game Freak, whom has a heavy hand in determining Pokemon content for Smash Bros, will want for the Pokemon Trainer to have Pokemon from another generation, thereby axing PT's current Pokemon.
Ivysaur:
Pokemon Trainer may be dropped since so many characters are needed to be programmed for his slot. Also, there is the possibility that Game Freak, whom has a heavy hand in determining Pokemon content for Smash Bros, will want for the Pokemon Trainer to have Pokemon from another generation, thereby axing PT's current Pokemon.
Charizard:
Pokemon Trainer may be dropped since so many characters are needed to be programmed for his slot. Also, there is the possibility that Game Freak, whom has a heavy hand in determining Pokemon content for Smash Bros, will want for the Pokemon Trainer to have Pokemon from another generation, thereby axing PT's current Pokemon.
Lucas:
A luigified-clone that is less popular than the original. The Mother series is finished, and two of its three games are Japan-only. Sakurai may opt to cut Lucas if space for the next Smash Bros. is limited considering that Lucas represents a dead series that is mostly Japan-only and the character itself is among the least popular newcomers introduced in Brawl.
ROB:
The reaction to ROB was about the same as it was to Dr. Mario, Pichu, and Young Link in Melee. ROB also the only character series not represented with a stage. ROB might have been placed in Brawl as plot device for the SSE. ROB is less likely to be cut than the other characters mentioned thus far, however, keep in mind that ROB was among the few Mario Kart racer veterans not to have been playable in Mario Kart Wii.
Note this list is not about characters that are "likely" to be cut, but rather characters whom are plausible to be put on the "chopping block". The Brawl newcomers whom have almost no chance of facing the chopping block are Wario, Diddy Kong, Meta Knight, King Dedede, Zero Suit Samus, Pit, and Olimar.
Lucario; well, he has a VERY Slight chance of retuning, namely due to him having a unique movestlye(his powe increases when hurt). but it's still like a glacier in Hell, overall.
Ike: either that or they replace him with Miciah, so they could have a new playstyle.
I don't really see any young incarnation of Link in the next game. it will probably be just Link by himself, sans mini clone this time. new Zelda charter slot for sure.
Wolf: pretty much agree here. if Falco is fox's Luigi, then Wolf is the Dr. Mario.
Snake. this pretty much depends on how they handle 3rd parties-whether they keep them or not is based on that. (this ai'n soul calibur, you know). at worst, they would just probably swap him for someone from castlevania.
sonic: same as above.
Lucas: in all honesty, i think Lucas has a BETTER chance of coming back then Ness. there has been not one but TWO cases of him nearly replacing the guy, and he took his slot as a default character in Brawl to boot. a lot of people say that he is safe since he's from the original, but Jigglypuff barely making it in Brawl proves otherwise. so yeah, this guy is actualy the safest of the bunch on this list.
ROB:if he was put in because of MK DS and not Gyromite, then he does actually have the Mario kart stage in his favor. i do agree he has a low chance of getting dropped.
PT: the pokes are debateable, but the Trainer himself is actually safe.
the only safe caharacters in the next game, going by the events and tradition, are:
the original 8(mario,link,Kirby,Samus,Fox,Pikachu,Yoshi, Donkey Kong)
the new 8(Diddy Kong, Pit, ZSS, Meta Knight, Lucas, Pokemon Trainer,Wario,Olimar)
-captain falcon
-Luigi
-(maybe) Falco
everyone else is suspect.