Thank for posting this up... it's a huge list so please excuse me if my suggestion(s) is/are already on the list as I have yet to read the whole thing...
- ability to pull up a friends list in the online lobbies so you can jump straight from one lobby to another that a friend or friends are in without the need to exit back to the lobby main screen with an option to see exactly the settings of the lobby before selecting to join;
- (probably already up there on the list, but...) absolutely comprehensive ability to filter in the online lobby main screen (pp from/to, bhp from/to, active steering on/off, race quality, connection quality etc);
- online lobby block list: ability to permanently block users from entering any/all lobbies that you make, with an option to unblock as well;
- invite option, to allow friends to quickly join you in a lobby and vice versa;
- menu option to disable the animations/waiting periods during oil changes etc in GT auto;
- ability to purchase paint chips separately;
- oil changes permitted in race lobbies/tuning sheet (online and offline);
- ability to change vehicle in offline pre-race lobbies, such as when in a single race, without having to exit back to the main race event screen;
- easier availability to earn credits - more seasonals, higher credit awards for existing races, more a-spec race events;
- 'rivals' feature which allows for a comparison between yourself and any number of other drivers in given areas of the game, including updates when a rival beats you in achieving a certain trophy or achievement etc;
- in online lobbies, the ability to see all members' fitted car parts (but not their actual settings, of course) including the ability to view each car's bhp/weight etc;
- bug reporting feature which allows for gamers to report bugs about the game direct to a handler at PD in a similar way to how one can send a report when the PS3 itself shuts down unexpectedly;
- ability to choose to voice chat with only certain members of the lobby, so as to facilitate both privacy and the possibility of team event racing without the risk of leaking tactics etc;
- unlimited tuning sheet save spaces (limited only by HDD space);
- ability to share tunes with other drivers (both friends and newly met racers);
- separate time trial leader boards for those who use driving aids and those who do not;
- mobile phone functionality in the shape of being able to send a b-spec driver into a race WITHOUT the need for the PS3 to be on, with progress updated/saved during booting of the game when the user eventually powers on their PS3 whilst connected to the internet;
- virtual equivalents of real-life racing events such as the Bathurst 12hrs and the le Mans 24hrs etc;
- ability to use all existing decals that appear on every race car in the game on any other car, as the user sees fit (seeing as absolute free reign will probably only lead to offensive designs being created anyway);
- ability to gift DLC to other drivers (at a cost, of course);
- ability to use one's own cars when restricting by single make in the online lobbies;
That's all for now. Again, sorry if most of these are already featured in the OP.
I'll add more later.