Rear rims have followed a fairly similar design for quite a while now. So unless something crazy and new comes along, I don't see that changing.
Stick to 135mm it works for all different sized wheels, one size is cheaper for bike and component manufacturers, it also opens up the choice in aftermarket hubs. I think from a design and materials point of view, that we've reached peak trials bike, it will just be incremental improvements.
Maybe more parts produced by additive manufacturing using exotic materials?