Whatever you want.
I always find limiting your narming convention to just one thing to be...limiting. And sensless. Is there really any sensible reason why?
This works for both ship classes and specific ship names.
Just look at us aircraft (F-4 Phantom, B-1b Lancer, F-14 Tomcat, F-18 Hornet, F-15 Eagle, F-6 Prowler, etc..) do they have anything in common? Nope. Should they? Nope.
Nah..let the imagination run wild.
Animals, mythical, real, immiginary, bird, insect, fish, mammal - everything goes.
Names of people, of places, of items - heck names of abstract concepts. Who the frak cares!
It doesn't even have to be a single word.
"Rain of Justice", "Pillar of Autumn", "Radiant Frost", "Song of Tranqulity".....EVERYTHING GOES... EVERYTHING.