Terra's end game might just be about him being saved from Xehanort's control, not really sure if he's gonna be "available" as one of the seven lights so soon that he'd be part of the final fight as a good guy (though he might help from within and fight Xehanort's control). If Terra is one, then Lea getting a keyblade has no relevancy. No, I think Terra's role is to be one of the baddies until the final confrontation and then he'll be saved in the end as they defeat Master Xehanort while maybe having some role in creating an opening in defeating one of the bad guys by fighting for control over himself with the Xehanort that took over his body.
I'd say the lights are most likely Sora, Riku, Kairi, Mickey, Ven, Aqua & Lea, though it is maybe possible some hijinks will go down with Sora and he becomes one of the baddies, in which case they'll need to figure out another light (which could be Terra but then MX would be down one of the 13 darknesses and that would need to be balanced out). The baddies are probably Master Xehanort, Young Xehanort, Terranort, Vanitas, Xemnas, Fake Ansem, Isa, Braig, a few other Org XIII members + maybe one or two surprises/new faces even (IIRC there's someone with MX's keyblade in Chi, maybe that's the first incarnation of Xehanort and will be part of the 13 darknesses)