Kingdom Hearts 3’s plot has already been teased by Square Enix, which revealed that both Sora and Riku will be on different paths before reuniting to defeat Xehanort. Of course, this is not caused by a feud between both characters but instead, they simply have different goals to achieve.
In Riku’s case, the lad will be accompanied by Mickey in his search for the lost Keyblade wielders. With that in mind, there is somehow a chance for Riku to pick up on Birth By Sleep’s plot and wake up Ventus from Castle Oblivion.
Ventus will then send Riku into the Realm of Darkness where he will have to find Aqua before convincing the latter to join the fight against Xehanort. If this is to happen, then it will also reintroduce familiar characters like Terra and Lea.
The former is required to aid Riku and Mickey in finding their way out of the dark realm whereas Lea will look to accompany Riku.
Then again, there is also the possibility for Riku to find Aqua first before being guided to Ventus. Of course, this will make the plot more akin to Birth By Sleep and might disrupt the natural order in Kingdom Hearts 3.
Nevertheless, the idea above sounds likely to happen for sure but only time will tell if this is true. Kingdom Hearts 3 is currently in its final stages of production and it is expected to make its debut at the end of this year.