I have a series titled 'Seldom Seen Ships of Star Fleet' that is comprised of odd, one off or low production ships. I imagine SF is like any navy or air force with low production, specialty or evaluation designs that get a lease on life but don't get the recognition of the more mainstream craft.
Here is a recently completed example, it's humorous backstory can be read here
https://www.deviantart.com/helot/art/Iotian-Class-969794914
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Seriously, I love the design. The back story integration is cool, especially since these ships were built by the Iotians and "gifted" to Starfleet, sufficiently explaining the design departure from other Starfleet ships.