Yes! It is a great idea... but, (there's a 'but' in every good idea) it needs to be done 100%. None of the one-foot-on, one-foot-off charades that they tried with S and P cars in GT5 should be attempted with this.
Basically, do it fully or let it be forgotten. If they do this though, it NEEDS to be online applicable. I can't even go on with how mindless the AI have been in GT5. If it was to be done, here is what I think Kaz should try.
Start off with making an open world--a city with huge sub-urbs. Throw in hotspots. Stadiums (Drift), Dragstrip, a race track for wager races. Give the world a limit to the amount of player vehicles that can roam. For example make a 100 car world, that way it isn't a "flustercluck" around every corner. If it were to be done, the world would need to be massive, but not short on details. GT5 saw what happened the last time the left out detail with the Standard Cars.
If it's to be done... it has to be the best there ever was.