I don't really know if I want to see another returning villain come into a later R&C game. With Nefarious' return it quite worked because he was a pretty menacing villain who wasn't defeated yet. It was a pretty big surprise that we saw him once again in the end of Quest for Booty and in A Crack in Time. Sure, it was great to see Dr.Nefarious in PS3 visuals who had the fun and B-Movie charm in him, but his plan felt kind of typical that was easily backfired near the end of the game.
As for the other villains, most of them would feel pretty pointless and out of the picture like the Space Pirates and the Thug-For-Less gang. And besides, I would move on with those group of baddies, especially the Space Pirates since the pirate trend should take a break at least, just like with Dr.Nefarious.
Some characters such as Courtney Gears and Ace Hardlight would have just moved on with their lives and perhaps changed their ways. They might not even be villains anymore, but then again, who certainly knows what those two are like these days.
As for Tachyon… I think he's pretty much dead since the way the boulder fell on top of him did look like that he didn't made it one bit. Although, with his small structure and appearance, there could be some possibility that he could of made that if he would of fit through some cracks and corners on mountains of boulders and rocks.
I really don't mind returning villains, but at the moment, i'd like to give this Artemis Zogg guy a chance, maybe, since Insomniac said they were going to bring back some PS2 villains, the Thug Leader and Gears are working with him. I highly doubt Gears just gave up, think about this, she hates organic life forms like Nefarious, and i'm sure that she would want to get revenge on Ratchet so it would work out. Ace on the other hand, I would rather see him team up with Ratchet rather than be a villain again, he could become a hero again. As much as I like the Space pirates, they should take a break for a while, Slag and Pete, I really like those two, perhaps they could be on the radio like in ACiT for gags, as they were already villains in two games. The Thugs is still a possibility, and I would love to see the leader and his army rise again, however, I don't think they are "great" enough to be main antagonists, so how about secondary antagonists. As for Tachyon, he wasn't crushed by any boulder, he was seen falling to the camera (towards the abyss) and was sucked into the wormhole and a rock near where Ratchet and Clank were standing was starting to fall apart, besides, the emperor was still screaming when he fell into the wormhole, so no he wasn't crushed by anything. From what is confirmed by Insomniac, that wormhole was opened by the Dimensionator, and Tachyon fell into it. Now going to Dimensionator logic, it could have just transported him to another dimension, however, the dimensionator was malfuctioning when it opened the portal. Getting back to topic, I would really love to see a female main antagonist, she would have to be a kickass hottie with weapons.