I don't know... I mean, I know it sounds strange, but Gran Turismo already have everything to make it perfect only if you mix all the details from previous games:
Gran Turismo 1
- separate music tunes for all segments of the game (including dealerships)
- laptimes and records tracked throughout the game
- scrollable list with all laptimes after the race
- strict restrictions for races
- SS11 with underpass
- GUI shortcuts
- 1-400, 1-1000, max speed tests
- Memory Card Battle
(2P races should be called like that for nostalgia reason)
- single-screen replay for 2P races (abandoned in favor of split-screen replay from GT4 onwards)
Gran Turismo 2
- greatest Time-Trial options ever (sector monitoring, 3 types of ghosts, etc..)
- best Race Analysis tool in the series
- ability to rearrange cars through Garage
- Tahiti, Midfield, Red Rock Valley, Rome, Rome Night, Apricot Hill, Grundenwald, Seattle, rally tracks..
- monitoring and tracking highest speed per venue
- Event Creator tool
- restricted career separated through "GT World" and "Special Event" races
- separated Arcade mode with Unlocks
Gran Turismo 3
- real-time mileage-monitor on HUD
- Time Trial Arcade races with predetermined conditions
- Complex String, SS11 with overpass
- when Original Tracks are raced in Reverse-configuration the lightning (time of day) changes
- save-slots for tunes
GT Concept: Tokyo-Geneva
> no real details except for the PodRace for lulz
Gran Turismo 4: Prologue
- Coffee Break tests
- License Tests structured with skill-tests
- aerodynamic/engine stall effect in high speeds
Gran Turismo 4
- stock-version of cars is unlocked for usage in Arcade/LAN multiplayer when bought in Dealerships/obtained through Prizes
- complex career with World Map
- Dealership Races
- LAN option
- old-school versions of some tracks, Motegi, Seoul, Hong Kong, Paris, Cathedral Rocks and El Capitan
GT5: Prologue
- leaderboards
- official matchmaking with predetermined entry-requirements
- pre-race garage sequences for particular track
GTPSP
- simple yet great Event Generator
+ all License tests were non-assisted in all GT games prior to GT5 > in GT5 many of the License test have SRF mandatory enabled and I personally find it insulting
Mix all above in GT5 with it's current assets/features and result would be stunning. Than enable changeable weather and day/night for all the tracks and I really do not need any other racing game for next 5 years.
BTW I probably forgot something in lists above, but those are details I could remember on the fly, without booting old games separately. If I did, I presume lists would be even longer.