It would be great with some sort of system based on skill, kind of how FFS would match people based on there ability which was achieved in game based on your race results while online.
It would also be good to be matched with your friends if they happened to be using the matchmaking system while you was. As we are limited to 100 PSN friends. it would also be good to have additional friends system within GT5, with no limit. So if you came accross a good racer, you could say befriend him within GT5 and possibly always be able to join a room he is in, or be matched with the matchmaker.
To get around this problem of racing with as many good guys as possible, without the need to be friends with 100 people, or for those 100 people to all be friends with each other so that it is possible for everyone to join each others Private Lounges I have done this.
https://www.gtplanet.net/forum/showthread.php?t=229785
using the above system, you only need to have a few PSN GT5 friends, but can have access to 100s racers via private lobbies that are always full. I run a similar private lobby set up and my lobbies are full every night with guys racing. All audited and all private.
If you want to go crazy, you could even make specific accounts/lounges for specific types of racing. like account. GT5_700PP for 700pp racing only etc (accounts never have to be online which is the magic)
Public lobbies are a no no for me. I pretty much always have bad experiences