I'll speculate.

My money's on Asus. Asus is a bit more mainstream than Nothing but still enough of an underdog that I think they should see the value in a partnership. They already target an enthusiast niche with the ROG line.

The Nothing Phone 3 uses an SD 8s Gen 4, which is not Qualcomm's "flagship" SOC, and it would be stretching the definition of "major" OEM, but who knows? This seems the most likely after Asus.

Moto's only flagship Snapdragon phone is the Razr Ultra, which I guess is possible. It'd be weird, but hey, I'd buy one.

OnePlus has been moving in the opposite direction for years now, locking things down more and more. I think they're too big for their britches at this point.

Sony's flagships are crazy expensive, well beyond the price of Pixels. They also don't cover the US market, though I'm not sure how important that is to the Graphene devs.

HMD doesn't make any phones with flagship SOCs. I think their best is the Skyline, with a 7s gen 2, Qualcomm's fourth-tier SOC line (the "s" stands for shitty).

Fairphone doesn't use flagship Snapdragons and GOS has had some pretty nasty things to say about them in the past.

Samsung is a pipe dream. They'd have no motivation. The entire GOS user base would be a rounding error to them.

On a global scale, Xiaomi would be a huge get. Not sure I see any of the Chinese OEMs focusing on this though.

Lenovo and Blackberry...might still exist? I think?