-More livery customization options (Like GRiD would be enough)
-More mountain like routes (Fictional and real-life (Transfagarasan, Stelvio etc.))
-More options in the track editor.
-Every track ever seen in a GT game+ All the F1 circuits (including Yas Marinas etc.), every DTM circuit,
-Create event option
-More cars. If I list every car I would like to see in GT5 the thread would increase 8 pages. I would like to see more modern cars from the year 2010 and 2011. But also classic's like those huge Bentley's, Bugatti's, the vintage 1920's ones
-graphic improvement on shadows, smoke, and lightning.
-More dynamic feeling to the tracks. I want to see more stuff happening. Photographs, security people, more fans and people standing at the side lines. Some more action.
-Racing action. I want to see more crashes. With full on damage. Cars flying trough the air. Like in real life.
-Even more competitive AI. They almost don't overtake each other.
-More DTM cars
-More hatchbacks
-Being able to choose tuning parts from different brands
-More tuning options concerning stream lining, bumpers etc. NFS Pro Street had this cool option were you could change the shape of a part.
-Better menu structure. I hate it to go trough all the menus all the time.
-GRASS, not just flat texture
-Detailed damage model. They should model a car like it was actually build like a real car. So they should make every part and then stick it together like a real car. With off course realistic physics. This probably would take ages to do.
I don't see this coming in the next 15-20 years. But once you've finish an engine, you can place it in every car that has that engine. Same goes for gear-box, chassis but also screws, bolts and everything. This would make it possible to recreate damage as never seen before.
-More tuning options. Being able to adjust the crankshaft, pistons etc.
-Invent something that recreates G-forces, vibrations, wind, sound etc. Some sort of ultimate racing(simulation) chair.
That would make the ultimate GT I think.