Gran Turismo
SPEC III
-AI
Let's be honest. since the release of GT5 (in fact, every other Gran Turismo for this matter) The artificial intelligence is brain dead. The passive and harmless CPU drivers gives the player no difficulty whatsoever. You can abuse the AI by ramming their cars and nudging them and they will not even try to aggressively overtake or bump on purpose. So spec III will, or should, have some kind of better AI, something that fights back.
-IMPROVED TRACK MAKER
When I saw that gran turismo 5 would have a track maker I was ecstatic. When I used it for the first time, it was pretty decent. but I wanted more features, like the point-to-point feature, noted in GT_Johan's post.
-POPULAR STANDARD CARS CONVERTED TO PREMIUM
Bugatti Veyron, Lamborghini countach, all four paganis, basically any popular standerd cars should get ye olde royal premium conversion.
-GRAN TURISMO TV
A small little store you probably noticed but forgot about. It has any movies about Gran Turismo and some buyable movies too, but honestly. what casual player would want to waste precious time there?!

the name is very misleading, so how about this: users can upload their own replays for others to watch and comment on it. It is similar to youtube, but anyone can use it.
FOLDERS IN GARAGE
How about a nifty feature that has been requested a few times? Creating folders to put cars in, rather than favorites, for easy access. for example:you create a folder labeled 'LMP' and you store your peugeot 908 and sauber mercedes c9 race car in there for easy access.
RACE TEAMS
The ability to create a racing team. You can search for racing teams and send applications via the Gran Turismo mail...option...that thing. When you create your team there is a one million credit opening fee. you can set the minimum A-Spec and B-Spec level to join and the featured car brand of that group. You can recognize team members with a star (★

by their name online. Every racing team can hold a maximum of 20.
BIGGER LOBBIES
How about a lobby of 18 people? 20? 24?
This is just my little list of what should reconstitute spec III. I believe that by building a stronger community net, rather than just competing in races, this will bring some positive changes.