I'm of two minds about the future of the series. I absolutely think that there is still a lot of story to be told, but I think RSB deliberately wrote TUC to be a sort of "ending just in-case", which is also where I think some (not all) of the ambiguity comes from. I do think certain things which are currently ambiguous will be resolved in TNG, though attempting to pinpoint where and what and how is a fool's errand. However I believe RSB when he says that "we're just getting to the interesting stuff". It feels to me like the entire series has basically been an effort to get to this point, where the No-God is walking and there are Dunyain controlling the Consult and Kellhus has been felled and Earwa must now actually fight the Consult openly in the manner we've been teased with in the Dreams and such. It seems to me that only now is RSB capable of "playing with all his toys" in the sandbox he has spent 7 books creating.
In fact, I think he did such good job making TUC seem like "the ultimate ending of everything" that some folks underestimate what remains of the story, and I also think that it's just hard for folks to imagine a story without Kellhus at the center of things (whether you like him or not), even though it seems clear that this has long been the plan of RSB and there yet remain many elements of the overall story to be told. Additionally, it seems to me that because we got generally got less metaphysical-type revelations than perhaps we expected in TUC (even though there are plenty, and TGO is chock-full of them), then that means there are no more to be had, which I believe is a bit misguided. While I fully expect to always be left with multiple interpretations and ambiguities, I don't think that immediately translates to there being no more revelations left whatsoever.
Really, I think one of the biggest things that makes it hard to get a signal for what is entailed in the next series is that Bakker himself does not fully know what is entailed for it, but clearly he has intentionally left himself more than enough plot elements to be be used as springboards.