I know there are a lot of things that need to happen before a lot of these scenarios play out. But nonetheless:
One issue I could foresee possibly happening revolves around the validator count. In complex systems, such as schools of fish or flocks of birds, it’s always those in the center that receive the most information. And that information then flows out towards the bounds.
In the future, information itself is only going to become a more valuable resource. And validators will be having the first crack at getting it within the Ethereum ecosystem. So, one problem I could foresee, is the validator count. Once the maximum amount of validators is reached (I believe the max validator count right now is around ~ 4 million, though I could be wrong) What happens to those that would want to be a validator? Would it be some FIFO method? Or some waiting list? It could be an interesting problem.