Mercer would love to be able to include some of the elements of the Tal'Dorei CG into what is currently released. As his wife is playing a Cobolt Soul Monk, you know he wants to put it into the official WotC books. Given the record pre-sales of the Wildemount book, I think it will go back on the table if it is not already in progress. Most likely, they'll release it as as Exandria Book that repeats some of the concepts from the TCG, but not all of them (so no feat to cast an additional 1st or 2nd level bonus action spell), but will spend some time focusing on areas that Vox Machina and the Mighty 9 have visited that werenot detailed in the prior books.
Nope. No need. There are two more continents on Exandria, so if WotC wants more product from Mercer, they just ask him to work on continent #3.
Yeah I doubt it. I won't say "never" because I didn't really think Wildemount was ever going to happen. But it doesn't really seem like something Wizard's, or even Mercer, would be that interested in doing. Rewriting a book that's already made seems too repetitive for both parties.
Surprise, it's in the Shattered TeethI think that Matt already knows where Camapign 3 will take place and will be building it with a new WotC book in mind as he goes. I'm betting it is Issylra, although I hope it is Marquet. He may surprise us and set it 50 years after Vox Machina in Tal'Dorei, though.