Riku is already confirmed to be playable, so it doesn't make sense his fight against Aqua to not be playable. Both will fight her.
It will probably go like this:
- Riku fighters her, but loses and gets his Keyblade broken.
- Riku will get a new Keyblade while Sora will save Aqua.
I believe Sora will succeed because in KH 0.2 Opening and KH3 TGS trailer 2018, we see his hand reaching her underwater. She will come to the surface simple and clean. We still didn't see Aqua with anyone from the Organization either.
It will probably go like this:
- Riku fighters her, but loses and gets his Keyblade broken.
- Riku will get a new Keyblade while Sora will save Aqua.
I believe Sora will succeed because in KH 0.2 Opening and KH3 TGS trailer 2018, we see his hand reaching her underwater. She will come to the surface simple and clean. We still didn't see Aqua with anyone from the Organization either.