There may have been a similarity at first, but when you look at it, MX and Terra fused. Riku was 'corrupted' but he chose the Way to Dawn and accepted the darkness in his heart. At least, until it was expelled in KH2 by DiZ's exploding machine.
Side Note:
In KH3 I can see Riku and Kairi being Sora's party members. Or they can cut out Kairi and put in Mickey. That'd be cool. And then there could be a 3-on-1 keyblade fight if Braig could get a keyblade. That is, of course, assuming Braig becomes the main-villain, and he tursn out to be a pawn of Xehanort.........again
So technically, he wouldn't be the main villain.
Wait, I've got it:
Braig is in KH3 because somewhere between BBS and KH1 his heartless was destroyed, thus becoming whole. He turns out to be full Xehanort's pawn. Somehow, Ven is released from Sora, becoming whole. All the keyblade wielders (Sora,Riku,Mickey,Van, and Kairi) somehow release Aqua from the darkness, and Aqua and Ven tell Sora the story of BBS. They then destroy Xehanort for good, and release Terra's heart.
In KH4 Braig returns, gains the ability to use a keyblade and becomes the main protaganist. This game will have 3 scenarios too, but be a console game with the scenarios being parties of S/R/K, T/A/V, and Mickey/Donald/Goofy. Thus they all fight through the game. T/A/V fight Braig, but he seperates back into a Heartless and a Nobody. M/D/G take care of the Heartless, and S/R/K take care of the nobody. Braig once again becomes whole due to M/D/G destroying the heartless first. Mickey is killed by Braig, and D/G return to the castle and go into retirement. Aqua goes into an all out frenzy and destroys Braig in a suicide attack. Everyone returns to Desiny Islands and Terra decides to put S/R/K through a Master Qualification Exam. That ends the game. Somehow this story would have to be the last of the Braig, or a stand-alone title between KH3 and the beginning of the next saga with a brand new antagonist.
At least I stayed on topic, putting Braig as the main villain