Unless his Skylanders appearance is meant to be sort of a cross-promotion, I doubt it. If Spyro hasn't had a standalone game in nearly eight years, then I don't see why Crash would get that treatment. Not to mention that Skylanders makes substantially more money than a new standalone Crash game probably would in this day and age.
I don't really get this argument.
While I personally like Spyro a bit more than Crash (growing up, it was sort of a Pokémon situation between my brother and I - he preferred Crash and was the expert of those games, while I was the Spyro guy), Crash is undeniably the bigger (unofficial) Playstation mascot than Spyro, partly because he was around a few years longer (Crash 3 was coming out when Spyro 1 was, and they both had secret demos for each other's games).
Crash returning as a Sony exclusive in his own game is a much easier idea to sell than it is with Spyro. He's just more associated with Playstation even to this day, whereas people more easily accepted Spyro going multiplat because the Legend of Spyro series in particular just had a better, more concrete direction than Crash's modern games and Spyro retained a distinctive following, even if it was newer fans.
Basically, at this point, if a new Spyro game were to come out, I think it would be multiplat anyway because people don't really associate him with Playstation as much, since he had a bunch of games that were still popular and well-received that were multiplat (LoS: Dawn of the Dragon, which was his last game before Skylanders, sold over a million). For the same reason, I don't think Sony would care to acquire exclusive rights themselves.
With Crash though, it's always been Sony teasing and referencing him. They know that the only games he has that people widely agree are good games, is the original Playstation trilogy and it's a much more iconic part of his identity than it is to Spyro, so re-acquiring him has an obvious benefit for their brand. For that reason, I think Crash has more reason to have a new game than Spyro does (even though I don't like how Skylanders absorbed him, the Skylanders games are still good games and Spyro is the main character of the Skylanders comics, so he isn't even in need of a revival either, he still exists and is relevant).
Correction: Raiden was picked over Snake to promote Metal Gear Rising, similar to how other third party characters were chosen (DmC Dante over Classic Dante to promote his upcoming game, Heihachi (who appears as a Mii costume in Smash 4 and was considered for a playable spot in that game) to promote Tekken Tag 2, etc.). Besides, Raiden already appeared as a sticker in Brawl.
That's the PR reason, but there was no way Snake would get in when Brawl was a thing. Too complicated from a licensing standpoint.
Also a sticker in Brawl doesn't mean anything, it's just a JPEG image of pre-existing artwork being plopped onto the disc.
With PSASBR, mainly the kill mechanic, I think what they were trying to go for was to have your opponent's all-star meter represent
your HP meter. Execution-wise, it doesn't work when some Level 1/2/3 moves horrendously outclass others. Still, I'd love for them to make an improved sequel, though they'd need to get a new studio to do it since, IIRC, the old one's since been shut down.
I feel like people also bash the roster pretty unnecessarily. Yeah, New!Dante exists, and having Good and Evil Cole as separate characters may seem weird, but that's about the only big red flags on the roster. That and people make it seems like it had a lot more third parties than it actually did.
That, and they'd need Lara Croft/Cloud/Crash/Spyro. Oh, and The Last of Us and Shadow of the Colossus content.
Actually, the developers of PS All-Stars did comment on SotC. They said they talked to Team ICO about it, but they said that they felt having their characters in a game like this would ruin the artistic integrity of those games, since there are games designed to be art and were never intended to be used for promotional shilling.
As for who would develop a sequel to PS All-Stars, it would likely be Santa Monica Studios (of God of War fame), because they developed the DLC for PS All-Stars (Superbot closed down before the DLC even happened).