I adore the characters of The Witcher world, especially those of The Witcher 2. But I came to terms a while ago that one casualty of the radical, daring open-world experiment of The Witcher 3 is going to mean that while our decisions from The Witcher 2 will likely not be contradicted, they won't be as huge as we hope. One of The Witcher 2's strengths is how variable the fates of the main cast can be. But the flip side of that means that many of the character realistically will only be referenced in letters or lore, especially if it means going to the trouble of making a character model and rehiring a voice actor for a character that is dead in half (or more) of potential imported savegames. One thing the Mass Effect games DID get right - specifically Mass Effect 3 - was making sure that every character you knew and loved from the second game showed up for a notable appearance in the third, and got more than just a single scene. Of course, ME3 ended up blowing it on a ton of other things, which is why it's not the greatest rpg ever, and just a pretty good game you should grab when it's 5 bucks.
But back on track. Essentially, I've gone through the important characters from Witcher 1/2 and rated both the likelihood of them coming back along with the necessity of them coming back (from a story perspective). Obviously there's some subjectivity here, but I've tried to consider as many perspectives as possible. I didn't tackle small-fry characters, especially those from TW1, since whether or not they return is much harder to anticipate since we don't know TW3's plot. Without further ado...
Likelihood of Return Ratings: Guaranteed/Probably/Possibly/Unlikely
Necessity of Return Ratings: Essential/Important/Relevant/Unnecessary
Triss, Yennefer, Roche, Iorveth, Dandelion, Zoltan, Vesemir: Guaranteed/Essential. All confirmed or strongly implied/stated by the devs.
Abigail: Unlikely/Unnecessary. Killable in TW1 and a natural wanderer. Enough women in Geralt's life as it is.
Adda: Unlikely/Relevant. Mentioned only in TW2 and her default state in TW2 is dead. Does not bode well. Likely a mention.
Leuvaarden: U/U. Interesting but pretty bound to Vizima, which won't feature in TW3.
Kalkstein: Unlikely/Important. An excellent staple of TW1, this fellow vanished oddly from TW2. I only recall the implication that the Kayran is one of his experiments gone wrong.
Eskel/Lambert: Possibly/Relevant. With Vesemir's return, we could see these two show up as his escort.
Thaler: Possibly/Relevant. Spymaster mentioned in TW2, so he stands a decent chance of showing up.
Yaevinn: Unlikely/Relevant. His role has been rather replaced by Iorveth in TW2, and he gets only a single mention. Given the fact that he can die in TW1 and didn't make it to TW2, it doesn't seem likely.
Siegfried: Possibly/Essential. A key compatriot of Geralt if you took the Order path in TW1, he also has a crucial role to play in the end of TW2. Dropping him would be foolish, and deprive Geralt of one of his consistent side characters.
Adam Pangratt: Unlikely/Unnecessary. He can perish in TW2, and he's not a particularly important fellow - just a merc, of which there are many more.
Anais La Valette: U/U. TW3 won't have her much older than she is in TW2, and most of the game states where she's relevant, she's spirited off somewhere to grow up safe and secure - whether with Roche or Radovid.
Mary Louisa La Valette: U/U. Given that she meets Geralt in only one path of TW1, and La Valette Castle is not going to be revisited, I'd bet against it.
Aryan La Valette: Possibly/Important. The Aryan choice is the big first choice of TW2, and his survival is important in the anti-Nilfgaard movement. He ought to reappear, perhaps in battle with the hated Black Ones.
Loredo: U/U. He lives only if you save Mottle, and players have pretty much tapped out Flotsam's story potential.
Bras of Ban Ard: Unlikely/Relevant. A cool cat of TW2's last chapter, it'd be nice to run into this fellow again, since he helped you get some of your best gear at the end of the game.
Dethmold: U/Relevant. Despite the fact that the only way for him to live is Roche's path/save Triss, Dethmold is an entertaining fellow who really would make a great random encounter mission where he tries to take vengeance on Geralt for mucking up his plans so badly in TW2.
Cynthia: Possibly/Essential. The most powerful magical Nilfgaard character players have yet encountered, Cynthia has a key role in TW2, and can survive most gamestates, unless you flatly decide to off her. Given the importance of Nilfgaard in TW3, it'd be a huge mistake to leave her out.
Henselt: Possibly/Essential. Henselt is one of the most memorable characters of TW2, and he lives through the majority of games - and is one of the two strongest rulers in the North. Crossing paths with Geralt one more time would make for a great conflict.
Natalis: Possibly/Essential. He survives all gamestates and is Temeria's only hope in TW2. Despite being 'only a soldier,' he's the most effective Northern general, and given the war setting of TW3, it makes a ton of sense.
Letho: Possibly/Relevant. If you kill him, he's dead, if you don't, he says he'll never show up again. Though we'd love to run into this charismatic meatbag again, The Witcher 2 is really Letho's story, and bringing him back could undermine the drama of Geralt's final choice of TW3 - especially if you just end up killing him, or having the choice again, for some other reason. And if he comes back and wreaks havoc, it will make Geralt's choice of mercy in TW2 look stupid. Better to perhaps leave him riding off into the sunset.
Philippa Eilhart: Probably/Essential. She cannot die in TW2, despite many places in the script where she could/would/should have died - in the prison cell, when she warps into an owl, later in Loc Muinne, etc. The fact that she is a guaranteed survivor of TW2 means this clever sorceress deserves a spot in TW3.
Radovid: Probably/Essential. As KOP will attest, Radovid is a crucial character to TW1 AND TW2. He can end the game as the most powerful Northern leader. He never really loses in Loc Muinne, ANY way you slice it. Radovid has an actual character arc and good development, we NEED to see where he ends up in TW3 as a result of his schemes - as well as the fate of the Order of the FR.
Renuald aep Matsen: U/U. Despite his cool role in TW2 chapter 3, he's little more than an Elite Mook.
Saskia: Probably/Essential. Another core character of TW2, one of the biggest decisions of the game is how she ends up - free, a slave, or dead. As the last dragon, she also is important in the fight against the Black Ones.
Sile: Possibly/Important. The Lodge's plans are wrecked, and Sile probably doesn't make it out of numerous TW2 games alive (assuming vindictive players.) She's not as clever or relevant as Philippa, so I can see her being offstage even if she lives.
Shilard: U/U. The Emhyr's orders to off him stand regardless of whether Geralt intervenes. He's not long for this world.
Stennis: Possibly/Relevant. Lynched or a King, Aedirn is going to be trashed by the Black Ones either way. I can't really see him having a huge contribution to TW3, but I'd like to see him nonetheless.
Ves: Possibly/Important. Her arc is incomplete. Roche will return, and so should she.