Here's a
possible idea for a personal ship for Cassy, I just came up with this about an hour or two ago, so I don't know if I'll go with this or not.
I was thinking, since The Doctor was UNIT's scientific adviser, I was thinking Cassy would be an adviser to Starfleet from time to time, and The Doctor got a car, I could see Cassy having a ship. Now I was picturing that Cassy's good friend, the mysterious Admiral Robert April (in my stories, he's extremely enigmatic, and been around before even Starfleet was formed. I'll leave it at that for now) was able to get for her an older hull, a destroyer, the USS Pegasus, that was just floating in some old depot, probably the same one we saw in "Unification parts 1 & 2".
Cassy then, in secret, went to heavily modify the partially dismantled hull. She added 2 more engine pods, and mounted 2 dishes. For the internals, she incorporated a lot of fairly new Starfleet technology, but also supplemented it Timelord technology she learned from the Doctor, as well as Dalek technology she back engineered, and some other odds and ends she learned during her own travels.
The warp drive is brand new, using Dalek technology, which is a bit faster than Starfleet's, and the ship has Timelord de-materialization circuit to be used for extreme emergencies where one has to get the hell out of some place. And it takes several hours for the mechanism to re-energize for another use.
The weaponry consists of standard Starfleet phasers, but also comes with Cassy's unique ripple phaser which causes damage across a wider arc. The torpedoes are standard make. The shields are normal starship shields.
The sensors and computers are what has been the most severely altered with the other technology Cassy learned. In fact, the bridge resembles more of a TARDIS console room than a Starfleet bridge. Also, apart from a small crew consisting of living, sentient macines, the ship is heavily automated. Cassy also jury-rigged some computer systems from several modern Starfleet shuttle craft to replace some of the aging systems her augmentations could not.
The ship ship is mainly for travel and exploration, but can hold handle itself in a fight when it needs to. Cassy is using the reverse of the Romulans when it comes to their ship designing. Where as the Romulans make these biggy, maxy sized behemoths, Cassy prefers to use deception, as in looking small and harmless, but in actuality being capable of giving an enemy on tremendous repair bill.
I'm just coming up with this stuff tonight, I might make changes here and there as time goes on, if I do keep this idea for my stories.

Here's some pictures of it. My bag 'o separate ship parts are limited right now, so once I manage to find more, I might do a lot more changes to it. I also would like to somehow put "C-Liner" on the front of the saucer, if it were possible.