An excellent idea.
I think the most important are more room restrictions/options and more search functionality. We should be able to fully define how the room will operate, down to hand picking allowed cars from the entire list and restricting each individual modification allowed for each car. Also, full control over ghosting/penalities/damage/etc in the race and during free run and qualify (we should be able to set as stand alone, or mixed with free run as in GT5).
In the lobby list, rooms should be searchable by title, host, and rules. We should also be allowed to set room options before creating the room to save time, and save as many configurations as we want so we can quickly recreate a room at anytime. Popular rooms could possibly even be shared with other players or displayed under a player's profile for advertising/recruiting.
We should also be able to create championships and endurances, possible with special rules like a fuel limit, or mandatory pit stop. Those rules should be available for regular races too, no need to carry on with pointless limitations. These can be open for anyone to join, or have a participant list managed by the host.
There is always more, but this is what was off the top of my head.