How should a future Spanish tech tree be handled?

Start with the ships and proposals that actually exist in the historical record, especially through the earlier and middle tiers. There seems to be enough material to build a substantial line without pretending Spain was a major battleship power.

The tricky part is the highest tiers. Reusing a design or introducing a more speculative ship might be necessary, but the game should be clear about where the historical material ends and creative design begins. That would feel better than stretching the history to make every tier look equally certain.