After Riku does battle with Nightmare Armor Ventus and Sora sinks into the pool of darkness and Riku dives in after him and ends up on Destiny Islands, he should have seen Sora sitting down at the shore and talked with him, with Sora mentioning they may be stuck there forever, then Ansem's bottle floats to shore, Riku reads it and the doorway back to the Realm of Light opens with Riku offering his hand to Sora saying "We'll go together." and Sora following it up with a smile and a "Yeah".

A callback to something that, in terms of continuity only JUST happened recently?

Why not? Ansem asked Riku his name in KH2 and he did it here too as a callback. So they could have done what was in my OP.

 

The fact it's pretty much the same situation but with Riku being the hero it was the perfect time for it.