If Motorola is any indication, Google will strip HTC of its patents and spin it off to fend for itself.
There is a big problem with Google acquiring HTC though - Apple will challenge the cross-licensing agreement they have with HTC and open the flood gates to the court room.
That said, Google needs HTC's VR tech if it wants to compete with Apple so if they can overcome the challenges of the acquisition, they would have an amazing VR platform of their own to take on Apple in the AR space.
As for the UI? HTC devices would keep their Sense UI and new devices would be AOSP or custom like the Pixels. Maybe Google will give the option to keep Sense or go to their UI to all current devices in the flagship support tree (HTC 10, Ultra, U11 most likely with all older devices deprecated). It wouldn't be hard to implement. The only thing that really holds back AOSP ROMs is HTC's custom hardware not having support.