Chack Would it be possible to pass along a couple of suggestions to the Motorola team designing a future GrapheneOS-compatible phone?
I think the time for that will be later. First the partnership needs to go well! Once it is going well, there may be room for hardware requests. But the timeline may not be short. Assuming everything goes very well with the Signature 27 series, that will likely be known some time in the summer. At that point the design for the 28's will probably be done, with initial construction and testing and certification in progress. So feature input, if it is requested, would plausibly be for the 29 series.
Meanwhile, I think it's important to keep in mind that the GrapheneOS user base is presently a small fraction of the world phone market. Internet rumor suggests that Motorola sells something like 3 million Signature devices per year. The GrapheneOS fleet is probably under half a million total devices (6, 7, 8, 9, 10). If all GrapheneOS users decided to sell their Pixels on eBay right away to buy a Motorola Signature 27 device, that would be a noticeable sales bump... but it would also probably mean very few GrapheneOS users would buy next year or the year after.
To be blunt, I don't suspect that Motorola sees the GrapheneOS community as an important source of data on how big camera bumps should be. What if all GrapheneOS users like bump-less phones, but everybody else likes lots of optical zoom and big bumps? I think that would suggest Motorola should make phones with big bumps.
I think Motorola may be hoping to grow their market share by attracting users interested in security, in which case that is probably where the value of the partnership will be.