Now that Sora's out of the way, Riku can finally take center stage.
The strangest part of Riku's role in the grand story of Kingdom Hearts is that he and Sora almost trade places in terms of their narrative arcs.
In the first game, the most significant plot twist is that Sora was never meant to wield the keyblade. The weapon was supposed to go to Riku, but latched onto Sora instead when Riku fell into the darkness. Sora assumed he was the chosen wielder, and became the hero of light with his actions. Eventually, through deeds, not destiny, Sora proved himself to be the better keyblade master.
That leaves Riku as the best friend and former rival. He knows that Sora is a better hero than he is, and he knows that he has a lot to make up for. Nonetheless, he wants to try to at least get to Sora's level. As kids, Riku was always the stronger and smarter of the two, but now he's in the opposite seat. He now knows what it's like to be brought low.
This gives his character room to grow and develop that Sora just doesn't have after the first Kingdom Hearts. Once he takes the mantle as hero, he basically stays in that position for the rest of the series. Meanwhile, Riku has to develop, make better choices, and become a better person just to compete.
Before, Riku was the chosen hero, and Sora the unwitting loser. Now, Sora is the hero, and Riku knows just how much he's screwed things up...
...which makes him better than Sasuke.