You can see the bike I just finished
here.
Here's some of my thoughts about custom bikes optimized for gravel (I won't call them gravel bikes since it makes people grumpy for some reason).
Wheels and Tires: I think you'll continue to see more clearance for fatter tires. There's also some interesting possibilities with 650b so I expect to see more customs experimenting with that size to get tire volume working in smaller frame sizes. At some point people are going to start looking at the aerodynamics and rolling resistance of knobs and we may see some interesting new treads.
Geometry: Bottom brackets will trend lower. There will be some experimentation with weight distribution and you'll see some bikes that are more rando and some that are more MTB with short stays and long front centers. Top tube slope will bounce around as people try to get a balance of frame bag capacity and seatpost flex (at least as long as that's a design goal). Trail numbers will be all over the place between rando and MTB too.
Brakes: Disc brakes will continue to grow in selection. Some people will stick to cantilevers in order to run more flexible forks. But interoperability with 29ers and other bikes in one's stable will be a compelling reason to go with discs.
Suspension: I don't think we'll see many full suspension systems, but I expect to see new takes on softtails, Softride stems, and maybe something like the Slingshot frames. Flexible seatposts should be interesting for a couple of years. The idea of 'good' flex might transfer to other parts of the bike, maybe a 'gravel' handlebar at some point.
Bags: I expect to see a lot more refinement here. Lighter bags that take the durability down a notch from 'Yukon Expedition". Better water carrying options. (a bladder in a Tangle bag puts the weight really high and right between your knees). Modern takes on handlebar bags. More integration - bikepacking style bags that mount to hard points on the frame instead of velcro everywhere.
Gearing: There's no way we won't be seeing more single front rings. I expect to see more subcompact doubles, something like a 30-44.