I don't think a Witcher game with CC will ever be as good as the ones we have had with Geralt. You just can't tell as good as story with CC as you can with an already established character. Geralt has so much back history to draw on for that, which you just can't get with a custom character. So, although I am not saying that I don't like making custom characters, I just don't think that I would ever be as excited about a Witcher game that didn't center around Geralt.
Well, I think you can... but that being said, I don't think that a customizable character precludes a pre-defined character, or a rich and detailed background. It didn't for Hawke or Shepard.
I pretend the books don't exist, so the scenario of the Witcher is, for me, contained within the three games I've completed. If we take that to be the case, then the extent of Geralt's backstory (not the particulars necessarily, just the detail upon which it is elaborated) that we receive in the games, particularly in light of the fact that he's an amnesiac for the first 1.5 games, would work just fine with a custom protagonist while still supplying a a strong character history. Honestly, with a few changes (albeit one or two large ones) it could straight up work with Geralt's backstory. Alter (or don't) a few of his past relationships, establish that women can, in fact, be witchers (this is essentially established by one of the endings of TW3, although I see no indication that they can't, other than the formulae for the correct decoctions being lost), and... that's kind of it.