When Master Xehanort began his travels around the universe, studying various subjects, he came across the Keyblade War. It was an event that consisted of a battle between Keyblade wielders of both light and dark. During this conflict, the weapon known as the Xblade was created. The same blade has the power to summon Kingdom Hearts, an omnipotent power that can pretty much mess up with reality as we know it. Xehanort, with ideals such as believing in a realm where everything is balanced between darkness and light, began machinating in his mind that, with that force, he could fulfill his wishes of a world where dark and light elements could peacefully coexist. That's when he created two plans;What I don't understand is why Master Xehanort really needs other members than himself filling the Organization.
A.) Take Ventus as his apprentice, divide his heart in two sides; light (Ven) and dark (Vanitas) and make both factions collide, forging the Xblade. Taking into consideration that his body had a short lifespan left, he decided that he should shift bodies (this is where Terra comes in). Slowly, but surely, he began deceiving Terra into falling to the dark, unwillingly, debilitating him, making a grand opening in his heart, making it easier for Xehanort to control. Everything went according to plan; he got control over Terra's body and forged the Xblade, but... The unthinkable occurred; Terra's Lingering Will went into his armor, knocking Terranort out cold, while Aqua (alongside Mickey), weakened Venitas (Vanitas controlling Ven) and Ventus could put up a fight, regaining control over his body, at the cost of the Xblade vanishing. During all this conflict, Kingdom Hearts was slowly appearing on the Realm of Light. But after the Xblade vanished, it let out a strong force that pushed everyone away; Aqua and Ventus into space, Terranort into Radiant Garden and Mickey somewhere where he was conscious and rescued Aqua and Ven, that after that event, Aqua hid him in Land of Departure and turned it into a maze; Castle Oblivion. After that, Aqua could feel Terra somewhere, she went to that location, but Xehanort took control and confronted her, almost killing her. Alas, she won, Terra regained control for a moment, trying to separate his heart from Xehanort's but, accidentally, wiped out his and Xehanort's memories.
In summation;
Plan a failed and this is when plan b comes around...
B.) Amass a group of light and dark Keyblade wielders to recreate the Keyblade War. There are enough light wielders (Yen Sid, Mickey, Sora and Riku (although they later grew in numbers)). Since there were next to nothing in terms of dark wielders, Xehanort had to create them. How? By finding individuals fit enough to stand the force of his heart and darkness, artificially creating dark Keyblade wielders to make clash against the light wielders, creating the second Keyblade War, forcefully forging the Xblade and summoning Kingdom Hearts.
In summation;
He forged a group of dark wielders to make them fight against the light wielders to recreate the Keyblade War, reforging the Xblade and resummoning Kingdom Hearts.
Up until now, it's been as simple as Ansem SoD telling Young Xehanort that he has time-traveling powers, making the other Xehanorts aware of the plan.Time travel (even as restricted as in KH) opens a door to all kind of bad story telling and inconsistencies,
I don't think Nomura is going to introduce another timeline, that would be too farfetched. What I think will happen is that, to end things peacefully, Sora and Master Xehanort will go back in time, in a period where Young Xehanort doesn't know about his time-traveling powers and both of them will try to persuade him to be good or fulfill what his old self wants. Since both Sora and Master Xehanort are complete opposites, they'd be acting like the conscious of good and evil.only alternate timelines are worse.
/edit
Riku acted as a good dark wielder for some time before he returned to the light. Just because the antagonists are on the side of darkness doesn't make the element itself evil. The reason Xehanort even is using darkness is because there are enough light wielders, if it were the other way around, he would go around spreading light and keyblade powers instead of dark."darkness is evil"
In summation;
It's a matter of circumstances when it comes to finding out darkness affiliation (good or evil), when in reality, they're elements, they can't be affiliated. I.E. Master Eraqus almost killed his son, his son, all because he was using darkness. Eraqus was a fanatic of the light, he couldn't understand that both elements can coexist. The only difference between light and dark is that light doesn't take over your body and darkness has to be tamed. '_'