This model has benefited PC games for years (of course the update were free). I just hope Sony does not charge us an arm and a leg for "new" stuff. I obviously have no problem paying for stuff like completely new tracks or cars, since that takes a considerable amount of time and effort. But leave stuff like standard to premium conversions, youtube uploading, other features, new events, and challenges free.
I could easily see GT5 going from what is frankly a B to B+ game (game standards, not sim standards) to an A. So much of the "core" of GT5 is perfect. The driving physics, tuning parts (w/ the except of brakes), car (premium) track modeling, weather, day and night, special effects, and even the games performance (yes it does slow down, but considering what is going on, its acceptable. The shadows are the only fly in the ointment).
Make the XP system and rewards a little more forgiving, add more events (old VS new Le Mans race, multiclass races, rivalry challenges, David vs Goliath, time attacks based on real life, etc), take some of the standard cars and make them premium (Audi R8LMS, Veyron, Furai, XF, some classic rally cars, and maybe a few others, I would say 100 total over the years) and finally add a damage slider OPTION for A and B spec. This part would be free (thats one hell of a free upgrade).
Then for the paid portion, simply add new cars and tracks (New Lancia Stratos, Lamborghini Aventador, Ferrari Hy-KERS, new M5, new Lotuses, Spa, Silverstone, maybe a new PD made track, or possibly upressed versions of GT4 tracks a la Trial Mountain). Make each car a dollar (lets face it they are not going to make that many), sell a bundle with a discount on each car and each track for $5. I think that is pretty fair.
Then when GT6 comes out it would have ALL this content plus, whatever else they add.