I would like to see new warships added that are intentionally more powerful and better than older ships to replace them.

What this does is it gives new players an incentive to build up something that is powerful that can significantly counter older designs. it also requires old players to play more actively to keep up.

I would say that the new ships should be 20-30% stronger than their predecessors. too much of an advantage would make people want to just sink their fleets, recycle, and rebuild mostly from scratch and do so effectively.