On the first...I have very little doubt in my midn that WotC won't bring over all the remaining Gygax Advrbture models into 5E at some point, and by all accounts I've heard the Wonderland stuff is Gygax at his best.
And they already talked about Gamma World explicitly on the DMgm a Gamma World...
I don't know if they will continue to keep it subtle and open to interpretation in this book proper, but the 3E obsession with bespoke cosmology for each Settibg just doesn't fit with D&D at this point. Eberron getting a distinct cosmology locally as a distinct expression of the First World...
Which fits well enough considering this is meant to pair with Rising from the Last War: that book had a lot of stat blocks that hold up well, and the campaign frameworks here are fairly humanoid-centric.
Interesting that we get both Llolthite Drow and the totally different Xen'drick Drow in...
They are on the same "Continent" sure...but in excess of 3200 miles away. It's greater than the distance between Sweden and India. Cultural differences between the Savage Frontker and the Eastern coast of the Sea of Fallen Stars will be extreme.
Fizban's and the new Monster Manual have moved away from direct parentage being the necessary cause of a half-dragon. Magic and draconic "radiation" are now listed as causes.