Just because of the potential visibility by someone/somewhere - I will focus strictly on functionality and options inside existing things in GT5:
Various Functionality
- oil-change inside pit-area
- Paint Shop introduction (buy paints already "unlocked" or bought through DLC for regular prices, sell paints you have/are gifted for symbolic amount of Cr)
- ability to manually delete times from Time Trial menu
- option "Remove All Parts" inside Garage which removes all removable tuning-parts
- option "Restore Default Tuning" inside Garage that restores all tuning values to default (for installed/default parts)
- non-mandatory SRF in License Tests and following that, ability to reset License Tests times and results once completed
Online Options
- oil-change in the pit-area
- ability to save "Lobby Preset" for Lobby options (either private or public)
- Lobby Theatre Mode (all people inside lobby can watch replay post-race together - lobby host controlling)
- introduce Host Tuning options:
None,
Host Tuning Parts Only,
Host Tuning Open and
Host Tuning Apply
- Host Tuning Parts Only - all cars in lobby have to have exact the same parts-tuning as Host Car
- Host Tuning Open - all cars can modify installed parts based on Host Tuning - parts needed/eligible to be seen through highlighting in Tuning Menu
- Host Tuning Apply - all cars will have tuning values applied as values of the Host car
This option is complementary with existing options of Part Tuning, where choice of Prohibited locks the tuning under values of Hosts' car (for same-spec racing)
- introduce Team Race - 2 players race for the same "team", post-race prizes are issued based on their combined results
B-Spec Online
Introduce the option to race B-Spec drivers in online multiplayer in the real-time (as regular MP option)
- players are in the Online Lobby (private or public)
- host chooses either A/B Spec race
- all players can sign one of their existing B-Spec drivers of choice
- track/car combo is selected, rules for race are selected
- race commence with real-time B-Spec management
- Team Race option - see above in Online options
- B-Spec XP + Cr for prizes
Gran Turismo 5: Prologue Heritage Races
Re-introduce official Matchmaking races in bi-week rhythm (to change when Seasonals change or can even be from the same category as Sasonals - but they're online MP races):
- 3 categories: Normal, Intermediate, Expert (as in GT5

)
- pre-determined set of rules (vehicle, track, # of laps, PP vehicle group, tyre compound)
- XP/Cr prizes only
Never forget:
Course Editor
- ability to create point-to-point stages
- ability to add chicanes
- patch the crazy algorithm of having Killer Blind Slopes on every major turn
> wishful thinking: add 4 more environments in the Course Editor
Absolute Wishful Thinking Category
Original Courses DLC with 5 tracks from previous GT games
(my personal choice would be Midfield Raceway, Apricot Hill, Grindelwald, Red Rock Valley and Rome Classic)
Thank you PD 👍