Battle mode, ala Mario Kart.
No, wait, there will be Ferraris, Lambos, and Porsches, but you can't drive them, they're just for show.
Oh, I've got it...to go with the pit crew there's an animated podium sequence where you get to kiss the trophy girl and spray the other drivers with champagne.
Hell, I'd just be satisfied with being able to tweak the striping and color schemes on the [R] mods, rather than some of the hideous freaks in GT and GT2 - but I have a feeling it's either something that we already know about (photomode?) or something obvious (cool secret tracks? cool secret cars? a recipe for muffins?).