Is it possible to 'roll-up' the Rollups?

OR (Optimistic Rollup) and ZKR (ZK Rollup) are great. But I'm curious,

Is it possible to build a ZKR on an OR? E.g. Polygon builds another zk rollup on Arbitrum

How about building an 'L3' on L2 to get 10,000x fee reduction? Or build an L4 and get 1 million fee reduction?? Or build an L5 and get... Well, you get the point

I guess it's possible in the far future, but for now L2s have to get lindy before scaling on L3. Ofc this is a blind guess, take it with a spoon of salt and pepper✌️