Only 61 years to go! (yes I'm going to keep using this joke forever)

There are so many possibilities. You could be a nameless hero from one of the races, you could be one of the main characters of the books. Though Anomander would be cool. Just imagine switching back-forth between dragon and tiiste form and each transformation bringing you closer to Starvard Demelain (or maybe timed). Then if not careful you're thrown in the warren and have to fight your way out. It would be a bit like the Bhaalspawn from BG2.

Why cant there be a TW4?

Are the Witcher books still being written?

My understanding is that the initial plans for the Witcher games involved a game in which you would create your own character, but that was dropped when they decided to center the games around Geralt. Which makes sense to me because it's probably more marketable that way. So in that sense, Witcher 4 would simply be circling back to that original idea with the comfort of knowing they can take more chances with their successful franchise.

2018-2020 is definitely not "soon" ... ;)

Considering the last CDP earnings report and their CEO said the game could be out as early as 2017, not 2018, I'd be comfortable saying soon. Not soon like next week, more like soon as in within a year or so.

Season of Storm (2013) was the latest, and it's kind of prequel. The story of Geralt officially ends with the last book of the cycle (Lady of the Lake), the games, comics etc are essentially "expanded universe".

So it's not impossible that more Witcher books appear (Season of Storm came as a surprise, at least to me), but they will probably flesh out earlier events. But I think it improbable, however much I would like it.

As for TW4... I think nothing is written in stone. While CDPR will probably need most of their manpower to make Cyberpunk a really great game, I could imagine that after a while people are drawn again to this compelling universe and its characters. While I think that Geralt's story is really concluded, there are so many possibility to examine other characters more closely. Letho's story until he met Geralt. Vesemir's youth...