I tried to boil down each ship to its base, to better give an idea of any "gaps" where a new ship could be inserted. These are just the first few things that came to mind when I was thinking about each ship; if anyone has anything to add, we can hopefully further refine our descriptions.
Goldfish: Forward firepower, good disabler, average stats. "Flying gun."
Squid: Hit-and-run, fast, lightly armored. "Fragile speedster."
Junker: Strong broadsides, good arcs, slow, moderate tanking ability. "Jack-of-all-trades."
Galleon: Powerful broadsides, limited arcs, little movement, good tanking. "Flying fortress."
Pyramidion: Forward firepower, limited arcs, fast, bad turning. "Vanguard."
Spire: Forward firepower, good arcs, slow, lightly armored. "Glass cannon."
Mobula: Excellent forward firepower, easy trifecta, slow, big blind spot. "Weapons platform."
I think any new ship would necessarily need to not easily fit into any of those descriptions if it was to stand out on its own. The trick is figuring out stuff that we don't have already, but would be interesting enough to add.