I'd really like to see the confirmation that Terra chose Riku and Ven chose Sora, because in the original Sunset Horizons video, Terra was the one wielding Sora's keyblade and Ven was the one wielding Riku's keyblade.
I really hope this has been pointed out before and I'm not the first, but it seems more like that when TAV chose their successors, it was intended for Riku to inherit the keyblade of light, but something must have mixed up and the keyblade of light was instead sent to Sora.
The keyblade of dark may have been intended for Kairi, but I think it's more likely that Aqua could not decide on a successor and was therefore not left for anyone to inherit but to find. And who knows if Mickey or Kairi is the rightful owner.
Please, this isn't a theory. I actually really want to know if anything has been confirmed yet.
For that matter, is it even confirmed that they actually CHOSE a successor of any kind? Were they even thinking of successors at the time? I mean, they're all apprentices of the true keyblade master at the time. Perhaps by the end, they themselves have inherited the three keyblades we know so well from the master and in the face of Master Xehanort they decide to take some precautions.
Again, not a theory, just thinking out loud.