Yeah, it's definitely easier for them to create expansions with new characters so they can save money on the voice acting. Basically just add more witcher contracts style of questing rather than dialogue heavy side missions with major characters.
But I've also said this quite emphatically in the past, it's on us to create reasonable suggestions in how to improve what CDPR already created. Small tweaks and dialogue expansions on existing scenes would be much easier than a full on character themed expansion or reworked epilogue.
However, all options should be suggested and discussed because we really don't know what CDPR is willing, capable and interested in doing. The more serious and well-considered options there are the higher the likelihood that something will be done.
I think the expansions will have to feature significant voice acting. I think trying to charge for DLC and then doing half baked cheap stuff isn't something i see CD Prokekt red doing as they were so cautious about actually starting to charge for DLC. So see no reason why they couldn't get her voice actresses back for expansion work. Not expectant but hopeful.
One small tweak that wouldn't need any dialogue would be the awful Yen kiss. Have her awkwardly hug Geralt and follow it up with Triss and Geralt kissing or hugging more passionately. Also change the codex so she comes at Geralt's request not Yen's if romanced.
If they are able to small dialogue tweaks, i would like some kind of dialogue on the boat when she's guarding avallac'h other than well. Talk about the upcoming battle, fears for what's to come, reassurance etc. Right now she's just to silent in the last act.
Think building her into Kaer Morhen would require more work even if that would be more of an ideal fix. Something small after the battle between Geralt, Yen, Triss and possibly Ciri dealing with the fall out from the relationship dynamics and at the least address the elephant in the room. It's silly that right now only really threesome Yen and Triss get any resolution of their personal issues with each other.