Redspear just talked about a similar ideas...
Yep, and when you consider all of those "step up from a Cobra mkIII" ships that have been made then there's lots of potential with ready-made ships should you wish for a progression.
Some possible examples:
Cobra mk III variants
(there's a slew of these, mostly slightly better but some considerably so).
Python Cruiser / Python ET
(regardless of the production dates it's your game so you can do what you like)
(one of the core ships but some might consider it a good candidate for such an approach)
(perhaps policing gets tougher later on in the game)
(a few variants have been made but bear in mingd that some of them may be attached to missions)
(a sort of super fer-de-lance)
Sidewinder / Asp specials
(I don't think they're massive improvements IIRC but another option)
Anything else you fancy.
They could show up in the spacelanes as well as for sale of course and I think I might know how to programme it should you be interested?