Not sure if this has already been said:
Track Meet: On GT4, you have this option in Gran Turismo Mode where you go to a track, select it and you aren't racing against the AI, but are putting down lapswhile they are there. Now imagine this with 15 other players and possibly an interactive garage, you can easily have a test session, like say a pre-test or maybe even a Qualifying session. I think it'll add a little realistic feel to people running an online series.
I've been thinking about something like that ever since PD were making noises about having an online mode, well before they released GT4. The option to go to any circuit for one of the track day/test sessions (that are known as "race meetings" in GT4) and hop out onto the circuit, where you may be joined by other drivers in different cars wanting to test.
My old idea also envisaged some sort of connection between all the "driving park" circuits. Obviously, you have the test course itself, the massive oval, but it would be great if you could pull off into the pits, and drive onto an access road which led you to the gymkhana, or Driving Park Beginner Course, or Driving Park Motorland - so you could spend time at one circuit and seamlessly drive onto another if you wanted to test different settings. It would make it amazingly immersive. They could even include more Driving Park circuits too, such as create a bigger test circuit along the lines of Complex String from GT3 and somehow integrate it within the testing complex.
All the while, users could join or exit when they pleased, and because of the size of the complex the chances are that the maximum 16 users wouldn't get in each others' way.
The closest games have come to the concept of a big testing facility so far that I've found (although without an online mode) are TOCA 2, which had various circuits, an oval, a dirt course and a skidpad all in one seamless area, and WRC3, which again had a testing facility with an oval, a tarmac stage, a gravel stage, an acceleration run, chicanes etc all in one, and you could race on different parts of it whenever you wanted.
The closest to my concept I suppose you'd find in the UK is the Millbrook Proving Ground in Bedfordshire. As you can see it has a banked circuit, a hill circuit, shorter circuits all with different surfaces and types of corners - everything needed for testing:
...and all linked by roads. Something similar in GT5 would be amazing.