There appear to be fairly massive additions to the map (the exact scale of the floating islands is unclear but it seems to be far more than can be viewed from the shots from ground level, and that's not even mentioning how the underground areas which we've barely seen), Link gets an entire new powerset, there's vehicle building and possibly weapon crafting (which seem to both be pretty central mechanics), the number of enemies and bosses looks to have basically doubled...

I really do not get this complaint. Yes some games do have DLCs that add quite a lot of substance, but even just what we've seen so far is clearly outside the scope of an expansion. Can you honestly name a DLC that adds and changes as much as we've seen is new in TotK?

Maybe if it only added like one or two regions worth of explorable area and one or two new powers and mechanics, but this is clearly a lot more than that.