It would take extensive work to achieve the hardware requirements and the capability for an OEM that isn't able to develop their own chips with Memory tagging enforcement (mte) which the Snapdragon 8 Elite used in the upcoming Motorola phones will have. It won't be cheap and it won't be easy.
Google has set the bar high with their security features.
It would need to provide that for around 7 years. It would need to avoid getting increasingly delayed over time. It would also need to provide the hardware-based security features required by GrapheneOS. That includes a proper secure element with the features we need and fully functional hardware memory tagging usable for all of the kernel and userspace. Motorola Mobility is working on providing what we need for next generation devices. Fairphone repeatedly expressed disinterest and is partnered with a company taking AOSP in the opposite direction from us for privacy and security. Fairphone replaced their own OS with that and it was a major downgrade moving them further away from possibly ever providing what we need.
Google has set the bar high with their security features.