Nose/tail art
Ship name
Engine fume colour
Decals
Engine tuning (are you flying an onionhead cobra? Make it sound batpoo crazy)
Ship light colour
Computer voice change - I'd go for a vocoder Cylon type ship voice (by your command)
Sponsor a station entrance message "family filtered ofc"
I'll add a few more as I think of them...
I would love to see some tasteful 'Cheesecake' nose art - which could be implimented as a specific decal location.
Ship name would be another good idea, and possibly a 'pay to change' sort of thing. This would prevent people changing the name of their ships on a whim, either to get past bounties or creep up on the same player they have been stalking.
More decals are sorely needed. Although I am not any rank in any faction, and therefore don't know if it already exists, but I expect there are plenty of people that would pay once for a "Faction Rank" decal set that they can upgrade as their rank goes up. PP faction decals would also surely be something that could be added to the game, perhaps also paint jobs [like the anime skins shown previojusly, but with the faction leaders on them.]
Computer voice, well you either need Majel Barrett [Star Trek, voice of the Enterprise computer and various Federation computers from 1967 to 2005, sadly passed away in 2008], Peter Tuddenham [Voice of Zen, Orac, and Slave from BBC's "Blake's 7" TV series, also unfortunately passed away in 2007.], John Leeson [voice of classic "Doctor Who" 's robotic dog, K-9], or Douglas Rain [Voice of the H.A.L.9000 computer from "2001: A Space Odyssey"].