It would be cool to see all of the characters around Novigrad, Velen or Skellige. You can randomly encounter Eskel and Lambert taking on Witcher contracts and you just run into them. As for Ciri, it depends on the ending, if she becomes a Witcher, you will randomly encounter her in different areas. If she becomes Empress, you can visit her in Nilfgaard, and maybe have Yen there as her advisor. As for Triss, she goes to Kovir, so not sure.
All of this probably won't happen, but would be cool.
Here are the solutions for Yenn/Triss/Ciri and the others with different endings:
1) A post main-story content with some dialogs, interactions and a few quests with main characters like: hunt, romance, drinking and talking with them even with a few dialogs, just to have the chance to talk to them. So basicly having them back in locations and mods @Witcher_Erune suggested that you can read in the post above with some interactions.
2) An ending quests that we can activate when we want, this will give us the chance not only to interact with them and do some stuff together but also Will Let us play secondary quests, contracts and exploring the world knowing that we are not alone like a dog, but we have our friends and beloved ones that we can visit when we feel lonely on just want to talk or see to them.
Also this option of the ending quests the we activate only if we want will also give us the chance to create our quests with main characters or using content created by other players. I know many don't like the idea of quests created by the others but IT will still be chance to install them or not, and this could make the game pretty much last for long time ( but this of mods is just a personal opinion).
3) After we activated this quests when we want, it will end the game, give us ending credits and finaly will let us start in a NG+.
Radovid lives: Change nothing!
Radovid dies: Remove pyres and witchhunters / add nilfgaardian soldiers
Dijkstra rules: Remove pyres and witchhunters
Triss (Radovid lives): At Kaer Morhen preparing to move to Kovir
Triss (Radovid dies/Dijkstra rules): At her old shop (preparing to move to Kovir)
Yennefer: At Kaer Morhen doing magic stuff (playing around with megascope etc.) or in a small house outside of Novigrad/Kaer Morhen
(If not romanced they are at the same locations, just without the romance option and/or different dialogues.)
Ciri empress: In Vizima castle preparing to be crowned
Ciri witcher: Training at Kaer Morhen or playing Gwent in an inn (option to do contracts with her)
Ciri never returns: Change nothing!
At their hideout preparing to leave it with different dialogues depending on the wars outcome.
At the Chameleon.
Change nothing! or at his hideout
At the Seven Cats playing Gwent.
Cerys rules: Place table in the "feast hall" (Kaer Trolde) where she's sitting and doing paper work. (A short conversation about Crachs death would be good.)
Hjalmar: Standing next to Cerys. Make it look like he's trying to advise her.
Hjalmar rules: Place table in the "feast hall" (Kaer Trolde) where he's standing looking on maps. (A short conversation about Crachs death would be good.)
Cerys: Standing next to Hjalmar. Make it look like she's trying to advise him.
Svanrige rules: Place table in the "feast hall" (Kaer Trolde) where he's standing looking on maps.
Birna: Standing next to Svanrige. Make it look like she's trying to advise him.
(Basically same location/dialogue/animation. Change only the position of the characters, depending on who became the ruler.)