I don't want "Darkness" to be the reason for Master Xehanort's entire plan. However, I would like to see Xehanort believe he was in control or even manipulating "Darkness" into doing his bidding.
I can see Master Xehanort being so dedicated to his cause, that he might actually put Darkness into himself, just so that he could execute the plan in the future. They can maybe even use Darkness as the explanation for the actual appearance of the Guardian (that Ansem has). I know it's supposed to be Terra - but why present that way, let alone help during the fight? Maybe they can even explain that Darkness was becoming too much of a burden for Master Xehanort to keep at bay, so he needed to dump it on another heart (Terra) while keeping Darkness inside himself, so that he could eventually do the split.
Yeah, basically I rather see Master Xehanort use Darkness as a tool in his crazy experiment/plot rather than have Darkness just lead Xehanort down some "destined path".