Disgaea has no chance. Disgaea only appeared once on a Nintendo platform and that was just a remake of the first game and it was on the DS.
Same situation for Phantom Brave. They are PlayStation exclusive except for 2 Wii games in 2009.
One of the Disgaea and Phantom Brave being in a Nintendo platform does warrant for these characters to appear in Super Smash Bros 4. Using Metal Gear series as an example, most of the games appeared in the Playstation platform, yet only about four of the said games appeared in the Nintendo platform (Metal Gear Solid: The Twin Snakes, Metal Gear Solid: Snake Eater 3D, Metal Gear, and Metal Gear 2).
Besides, it really helps Nippon Ichi to get more recognition (along with the *shrug* sub-only anime that the American division licensed) if the Nippon Ichi characters appear in Super Smash Bros 4. Besides, like I said, none of the Nippon Ichi characters ever appeared in a fighting game like MvC3 or Super Smash Bros.
Zero is not happening. He's not any kind of mascot for Capcom. Ryu, Mega Man Classic, & Mega Man .EXE are the only characters they could even pitch to Sakurai.
Explain why Zero was chosen over the original Megaman in MvC3? Apparently, in an interview with Ryota Niitsuma, he said that Zero had a more variety of moves than the original Megaman and if they were to include the original Megaman in the fighting game, they would have ended up putting more moves in Megaman that would have created an entirely different Megaman. i think Sakurai would use his similar reasoning to choose Megaman Zero over the original Megaman. Besides, I believe Zero have a sizable amount of fan support comparable to the original Megaman (or even larger).
Kinda hate to say it, but Knuckles kinda died along with most of the other characters. Sonic Unleashed, Colors, Sonic 4, & now Sonic Generations, Knuckles is nowhere to be found. If they do give Sega another 2 slots (unlikely) it will probably be Tails and then a character from a different series.
I think Knuckles appeared in Sonic Colors. I also believe there are two more episode installments of Sonic 4, so appearance of Knuckles as a playable character may be possible.
Funny thing is that the appearance of Marth and Roy in SSBM was the main reason the
Fire Emblem games got an international release. Now I was wondering if there are characters from Japan-only games that can be put in Super Smash Bros 4, and maybe we might get a similar effect. I think the only way Roy can come back in Super Smash Bros is if there is a remake or anime adaption of Fire Emblem: Sword of Seals.