So here's the core idea. This draws from david wford's idea for logged out fleet members as Bridge Officers, my idea for guest stars (which I chalked up to Kingdom Hearts as a partial inspiration), and Kestrel's comments about how she can't assign personalities to bridge officers.
The core idea is this:
You have prefab NPCs with a variety of distinctive personalities, which may even include canon characters like Worf or Miral Paris or Joshua Riker.
These NPCs are unlocked by a fleets through special upgrade projects using Fleet Credits and have distinctive dialogue.
Once unlocked, anyone with sufficient privileges can "assign" these NPCs to various Starbase roles such as "Officer of the Watch." When assigned, the Starbase NPC gets some added flavor dialogue and takes on the costume of the NPC. There may be other mild perks such as these FOs' traits applying to all fleet members.
Additionally, these NPCs may be utilized by Kestrel for flavor dialogues in missions, with the provision that she'd have to dance around the idea of whether or not they're with you. And players who are members of a fleet may assign Fleet Officers to bridge officer stations or away teams, acting as "shared" bridge officers.
Fleet Officers come totally prefab and cannot be retrained but have perks like unique skills (exclusive ranks of skills or perhaps even a mix of class skills) or unique traits.
You don't "own" them but anyone in the fleet can use them as though they had these characters as bridge officers. They're effectively fleet-shared bridge officers.
Cryptic can release variants of them ore desirable canon characters in doses as limited time projects, like holographic Scotty or, say, during the Winter Event, projects open up for Winter garment versions of the characters and during the Summer Event, versions in unbuttoned uniforms or even swimwear with flavor dialogue about the life support systems making the temperature warm or about how they just got back from the holosuite.
So perhaps Worf is available for Klingons year round as an expensive unlock but around Christmas a discounted "Worf in winter clothes" opens up.
Additionally, holdings like embassies could have the added perk of unlocking additional Fleet Officers of other factions (ie. maxed Romulan Embassy might unlock Romulan Diplomat FOs whereas a Dilithium Mine might unlock Reman and Cardassian FOs).