At the end of 358/2, Roxas was on his way to go and kick Xemnas' ass and end the Organization once and for all. Prior to this, he'd annihilated Saix, proving that he was more powerful than the second ranked member of the Organization.
On his own, Riku couldn't defeat Roxas and had to rely on the Darkness in his Heart to catch Roxas off guard. As demonstrated in KH2, it took Riku and Sora to defeat Xemnas without Riku using his Darkness.
In Final Mix+, Roxas defeats Sora in a fight (Need confirmation of this).
From this, we can surmise that Roxas is more powerful than Riku and he is also more powerful than Sora.
So, if Riku had never stopped Roxas, would Roxas have defeated Xemnas? How could this have affected the storyline and mythos? Thoughts on this bat***t insane theory?