Anyway, the POD here would be GRRM being dissatisfied with Benioff and Weiss, and pulling a "
Fine, I'll do it myself". The result would presumably be that he'd instruct screen-writers to adapt his books into script format pretty directly, with few(er) deviations. Less focus on "sexposition", one may assume. Lots of elements that were whittled down in the OTL series would remain in this version (such as Lady Stoneheart), and the general pace of the series might be perceived as slower. Considering that we
know that Benioff and Weiss wanted to speed things along (especially near the end), I would guess that a version made/guided by Martin himself would consist of ten full-length seasons. (One for AGoT, one for ACoK, two for ASoS, two for AFfC/ADwD and two each for TWoW and ADoS.)
This would all be happening well after
A Feast For Crows was good and published, so it doesn't radically change the plan for the books. But it would slow down the writing there, presumably to "next to no progress".
A Dance With Dragons might not make it to publication, and everyone can just forget about
The Winds of Winter. As
@GSD310 wrote: ASoIaF would become a saga that began on the page, and ultimately culminated on the screen, without the written version ever likely to be finished.
Someone else finishes the show when he’s incapable of doing it. GRRM is totally unsuited for Hollywood speed of writing.
Being good at writing books doesn't necessarily mean you'll be good at making a movie or a TV show. Just look at Stephen King and Maximum Overdrive.
He wouldn't have to be the writer, although the man actually has a pretty extensive career in both writing and producing for television. His role here presumbably wouldn't be writer, though, but lead producer with final script approval (and an occasional role personally re-writing a few bits or serving as co-writer).