There's a wishlist of what a future GOS device should support here:
https://grapheneos.org/faq#future-devices
That makes sense as a justification of why not support anything but Pixel. But if Pixel is potentially off the table, it needs looking for some other silicon vendor. It's likely not possible to achieve everything on the list. So which vendor is the next best out there? If there's a preferred vendor then it's possible to look for handsets or possibly ODMs using them.
Before anyone asks, DIY silicon is not happening with GOS volumes. While I have some existing experience and maybe IP to contribute in this area, we're talking $200m+ for a competitive phone chip. Not happening. Doing a new phone with an existing chip is a much lower barrier to entry and might be more plausible, and a custom board or customized firmware could potentially fix some of the non-SoC requirements on the list.
We know why vendors X/Y/Z are not as good as Pixel, but who is a plausible next best?