Maybe it makes sense from a business perspective, but it's debatable whether or not it makes sense from a customer perspective, since many customers (including myself) would be against a tacked on continuation.
From the forums, many many more fans are for a sequel.
There's simply nowhere for the story to turn at this point, all the enemies are defeated, Ciri killed the physical manifestation of all entropy across the multiverse somehow, what could a Witcher 4 be about?
Are you Michael Bay? Must everything be bigger, badder, with more explosions? If not, then the next enemies can and should be something more personal.
Suggested possible enemies off the top of my head:
Philippa Eilhart
Emperor Emhyr (who would have faked his death in an assassination ending)
Avallach
The Last Crone
The Disciples of Vilgefortz attempting to resurrect their master
The new Aen Elle monarch
The Order of the Flaming Rose
Witchers of the Cat School
Or, god forbid, they actually come up with a new enemy.
Possible Locations:
Nilfgaard
Cintra
Occupied Temeria by Nilfgaard or Nilfgaard supporting Temerians
Zerkania
Rivia
Lyria
Kovir
Ciri being a self-styled Witcher when her only Witcher-like abilities are being able to kill things with a sword?
Ciri's abilties:
* Replace Signs with Slow Time, Extra-Hard via Flash Step attack, Teleport Behind, and Freeze Enemy
* Ciri can make Sorceress Potions, Amulets, and Oils just fine
* Yes, swordsmanship
Or maybe God himself shows up and everything is hopeless until Geralt pulls an android 17 (or vesemir...) and gets jobbed/dies to provoke Ciri's inevitable powerup to save everything, again.
Given the Last Crone has Vesmir's amulet, I assumed we'd see Vesemir's Ghost summoned with it at some point. Perhaps giving her a weapon against Ciri.
No thanks, she's too OP, there would be no prep work, it would only be fan fiction tier writing at this point.
Geralt is the greatest swordsman in the world and also a superhuman mutant yet gets his ass kicked by Bandits. Ciri can teleport and is the 2nd greatest swordsman in the world so she should be fine.
The only continuation of the franchise I could see not being terrible would be a prequel of some sort, either as Geralt or if they really want to shift characters for some god forsaken reason, Vesemir in the Witcher hay day.
Geralts story is done but I wouldn't mind him having a supporting role in the next Ciri game.
---------- Updated at 07:47 PM ----------
Plotlines for a Ciri game off the top of my head
* Ciri is being hunted by the political enemies of Emperor Emhyr to destroy his bloodline.
* Emperor Emhyr is actually evil (surprise!) and wants to take her child to rule the Empire in her place w/ her as his puppet.
* Philippa Eilhart wants to take control of Ciri's mind to rule the Empire behind the scenes like Saskia.
* The Prophecy of her child ruling the world is still unfulfilled so people still want to force them to marry her.
* The Third Nilfgaard War is not done just because the tide is done and Ciri can try to take over.
* Cintran Nationalists want to restore Ciri to the throne of Cintra, independent from Nilfgaard and lead a revolution against them.
* The Lodge of Sorceresses want to restore Ciri's magical abilities so she can become a proper sorceress.
* The Cat School has been hired to hunt down and kill Ciri. They consider her claim to be a Witcher insulting to their order.
* The Last Crone wants revenge for her sister.
* Avallach has decided that he is going to use Ciri to become ruler of the Aen Sidhe.
* Lara Dorren's ghost has been placed in the body of a young white-haired woman and placed on the throne of Nilfgaard as a Fake Ciri. She orders the destruction of Ciri. It's at the hands of the Disciples of Vilgefortz who are secretly controlling her ancestors' mind.
* The Order of the Flaming Rose is seeking out Ciri because they believe her child is the destined successor to Jacques Aldersberg and their leader has an insane desire to impregnate her. They refuse to believe the White Frost is defeated.
* The Eternal Fire has declared Ciri to be their version of the Anti-Christ and started a hunt throughout the land for all girls vaguely matching her description.
* Anais is now a teenager and has decided to lead a revolution against Regent Roche in Temeria. Ciri can help her or fight her.
* A rabidly racist Imperial Marshal has decided he is going to destroy all nonhumans in the kingdom and Ciri must stop him.