I always been wondering.......what if they switch roles? Instead of Sora getting captured by team Xehanort, they get Riku instead. What would happen?

One, they tried in the beginning but couldn't bc he had learned to control and overcome his darkness so they ended up not being able to use him. That lead to going after Sora.

 

But seriously if they had succeeded it would've been lame bc we have already witnessed Riku's story arc of being captured by darkness and having to overcome it. DDD was his redemption and his story to show how far he has come.

He'd be possessed by Darkness AGAIN (I think he gets a free smoothie by that point), Sora would have to save him AGAIN, and Xehanort would be all high and mighty over how he won over Riku to his side AGAIN. Basically, it would have been a lot of the same that we've been exposed to already so thankfully they gave us something different.

I always been wondering.......what if they switch roles? Instead of Sora getting captured by team Xehanort, they get Riku instead. What would happen?

It's already been stated that Riku is now useless to Xehanort having built up a resistance to Darkness.

Then it would have ended with this. 

 

I just want to see an evil Sora with yellow eyes that isn't Vanitas tbh. I want to see Soranort REALLY bad, even if it's for a brief few seconds. Or possessed Sora at least.

Please refer to Kingdom Hearts 1 despite not having The World that Never Was.