I'm new here, but I've been playing GT since GT4 and I've always wanted DLC...so here it goes.
Tracks:
Former tracks from all GT games that didn not make it into GT5.
Cars (where do I begin?):
More Classic Muscle Cars (USA)
More Trucks, perferebly older models since new modles are fequent
More premiums
Upgraded premiums from current cars in the game
More cars like X2010, similar, but not as good
OTHER (some of these are patches and things I'd like fixed):
Open world I guess. Having your cars and being able to go anywhere you wanted (in a contained enviroment. Would be cool if it was actually a real place). Ablility to get out, walk around, ability to stall cars, ability to shut off cars, ability to have to have to pay for gas, tires etc (more realism, blegh, don't flame me for that). Ability to have debts (of course not insane debt but just enough to get you grinding for cash)
Extremely better course maker
More tuning mods
Ability to pop the hood and tinker with your engine (or at least view it)
Ability to open doors and look around
Ability to pop the hatch for storage
Ability to purchase and use nitrous (like in GT4)
Ability to add more customization options (even though people frown upon vinyls, underlights, diamond plates, etc), would be cool to have
Ability to mold custom bumpers for customization
Ability to add Pickup Bedbars with offroading lamps (for dirt/snow tracks at night, more light to see course)
Ability to change wheels on standard model cars
Ability to paint wheels on standard model cars
Ability to save mid race, on any race (I know, it's coming soon)
Ability to race against more players online (32, like karts, instead of 16 for cars)
Ability to have to change small things (spark,plugs, wires, batteries, starters etc)
Drop prices on things, especially if new things such as stated above are added
More wing and aerodynamic options for cars that don't have any
More wheel options (like GT4, it has a whole store itself in GT Auto)
Fix the car was people (half the time people washing the cars in GT Auto miss the car and are scrubbing the air)
Fix GT Auto wash for karts (there is no one)
Paint Preview
Oil change on karts
Ability to fine tune carts
More photo destinations
Faster load times (don't we all want this? PS3 is more than capable of doing it)
More races, in general
More seasonal events (updated every 2 weeks)
More remote race options
Ability to speed up B-spec races (like GT4, fast forward up to 4 times faster that real time)
The old garage or similar from GT4
Completely redesigned main menu in GT life (NEVER EVER GOING TO HAPPEN)
More trophies
Faster online loading times
Redesigned main online lobby
Redesigned lobby once in game
Ability to rename lobbies
Ability to set fixed host status when aquiring a lobby form another user
Ability to set mic usage when aquiring a lobby from another user
Invite system
Easier way to find how to borrow cars
More car restrictions
More event restrictions
More regulation restrictions
Ability to turn ABS off in some areas where it can't be
Sixaxis support (never happening now)
Better sounds (engine, etc)
More sounds
Skid marks created at start of race to still be there at the end of the race
Returning music from past GT games
Ability to host your own events and award prizes (money, gear, horns, paint, etc.)
Ability to sell items and cars online, in the online dealership/online store
...as I kill my brain trying to think of things that need fixing and DLC I want
Not really DLC here, but a ton of patches to fix everything that wasn't completed at launch (like better damage, even though some cool stuff is unlocked at level 40), along with new features.
It would be really neat to see Polyphony and Kaz do an offraoding game like Atv Offroad Fury. Maybe even get some classic 3 wheelers in there
Think that about does it =P