A good idea. Perhaps this could be implemented by creating a 'Beast' race, with specific techs. and AI behavoirs, and only ships. (No planets or colonization tech.)
Advantages: You could include or not include that race depending on prefences, and it would closely coralate to the old space monsters from MOOII.
Disadvantage: I'm too new to even think about how this could really be done.
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eSnaffu