With proof of work blocks, the difficulty is increased as long as the network is putting out proof of work blocks too fast, which works to set the average time between proof of work blocks.
Proof of stake is a more deterministic process, so I don’t see that working the same way. “Difficulty” could be a maximum stake that’s required, but if many people meet it then it doesn’t stop them from all claiming it at once the moment after they receive the previous block, and if no one meets it then the network fails to get any blocks/confirmations for a while.
The difficulty could ramp down with time after the last block, but I’m not sure how closely you can enforce time; would blocks that were timestamped an hour in the future for the lower difficulty be accepted? Maybe if the nodes all had a rule to ignore blocks (more than X minutes) in the future, but that still seems to give you a bit of wiggle-room which bothers me.