Indeed. As you said, given everything included in GameStack, I would not be surprised if IdTech is worked on as something that can be attached to GS to broaden the appeal of their offering.
Interestingly doom eternal already uses playfab so there is already some integration there, it also uses havoc too.
The only major thing that gamefab is missing is an audio engine imo, and the only major one that is not owned by a larger company is fmod. Having a dedicated procedural materials/objects tool would be good too.
It will be interesting to see if microsoft keeps acquiring middleware companies, especially with the middleware market heating up with acquisitions, with epic leading the charge