HTC was a good fit as it is a quality OEM and the partnership was good for HTC who has been stumbling as of late, but I suspect that as long as Google "partners up" it will bounce around to avoid showing prejudice towards any one in particular.
That said, I'm hopeful that Google is aiming to get away from using HTC, Moto, LG, etc for their manufacturing and directly contracts with someone like Foxconn or Flex Ltd who will be able to truly build a Google design phone from the ground up and have the capability of better scaling to demand (which could be a conflict for a contracted competitor, if boosting Pixel production would hinder their own), which if they ever get around to innovating their own processor (which, in turn, would empower them to guarantee more than two years of OS updates - something they cannot do currently as they are at the whim of Qualcomm's option to ensure support or not) would be a logical decision.