In a perfect world, where fish were not endangered, in my opinion, the fish for fish pie should be two thirds fresh cod and one third smoked (Finnan) haddock.
The haddock is marginally less endangered than the cod, so you could substitute fresh haddock for fresh cod, but it doesn't have quite that chunky translucent flakiness that cod has.
I don't think that prawns, anchovies or capers are improvements.