By unique ship, I mean one that is limited to one per list. Right now, you can fly two YT-1300s, but only one can be the Falcon, or four HWKs, but only one can be the Moldy Crow. But why not a ship that is truly unique? Star Wars has plenty of examples of prototype ships, extreme custom modifications, and the like to support the fluff of such a ship, and it'd allow FFG to play with some interesting design space without worrying about players just fielding a list of multiples of the same ship.
However, the cost of developing a whole ship that mechanically restricts itself to a single purchase might make the idea unfeasible. Even if it came with killer upgrades, it'd be incredibly hard for anyone to justify the purchase of a second ship when you can literally only field one.