1. Ciri is the obvious choice but, because of the lack of Witcher mutations you loose so much of the mechanics of what make the game great and unless every other female NPC in the Witchers world are Bi or gay then another majour aspect. As much as I love Ciri as a character, this is my least favourite.
2. Then there is the create your own Witcher from your school of choice which could be a prequel or a sequel. Can't see much wrong here apart from loosing the great dialogue in these games and some lore smudging.
3. A young Vesimer set a few decades prior, one where you could even have some current or even dead characters appear. This is probably my favourite of my three ideas as it require very little change to any aspect of what The Witcher 3 eventually perfected and there is plenty of lore for CDPR to work from without scewing to far away.
And then there is the obvious return of Geralt after the events of B&W where he gets gets pulled into a greater story, perhaps one where the lodge goes after Ciri or Emyher hunts his heir. Perhaps the true white frost does come and Ciri plays alongside Geralt in a world hopping adventure to rid it entirely. Hmm, The safest bet but perhaps the most boring considering all that has been exhausted in the games so far.
Anyway, enough of mine, this thread is to hear your ideas and give us all hope that there are parts away from a Ciri led sequel.