kh1 started with sora.
kh2 started with roxas.
kh3 will start with riku, (because sora fell to darkness.) I mean cmon it has to happen. Yeah he will come back to the light, but he has to fall at least a little (to make some god dang character development.) It makes sense to me.
I couldn't see the game starting with Riku. Riku isn't nearly connected to Sora on the level that Roxas is. However, I could see the game starting off on the Station of Awakening as usual, but it would be Sora's heart this time and it would partially be consumed by darkness. You would start out as Ventus for this reason, who is part of Sora's heart trying to fend off that darkness.