I doubt we'll play as Lea but his role is to save Roxas (and Xion even if he doesn't remember her right now). How he will do this and what other things he might do in the game...well...
I have this strong hope for Lea & Kairi being a ginger duo during KH3 where they train and travel together and do all of the leftover stuff in the plot. Since Riku & Mickey are off looking for Keyblade wielders, specifically TAV, and SDG are off on a wild goose chase for the Key to Return Hearts, that leaves Lea and Kairi to go solve mysteries and become bros.
They both have red hair, both have idiots for friends that like running off and doing things on their own and carry the emotional baggage and...and they can hang out on the Twilight Town clocktower and eat ice cream and talk about their problems.
Pushing the potential awesome Lea-Kairi friendship aside, Lea's always been a top-notch sleuth and hitman. I would imagine that he'd be a great help to the crew in discovering potential leads on Xehanort's whereabouts. He'll also probably spend plenty of the game dealing with Isa/Saix and the emotional turmoil from any encounters.
Lea is also one of the few people who possibly knows what went on in the laboratory the day Xehanort went psycho, and there is still the matter of Axel and Saix's plans for a coup. We need to find out what happened to Lea & Isa, why they wanted to plan the coup/their goal, and see just what will become of their relationship in KH3.
If Nomura wants to try and make us question Lea's allegiance to the heroes (Nomura likes drama, Nomura likes mysteries, Nomura likes attempting to pull the wool over our eyes) I wouldn't be surprised if we were to see brief scenes where Lea and Isa/Saix interact but we don't hear much of it, which will lead us to wonder who is what side and who should we trust. I can seriously see the magazine scan taglines now: "Lea & Saix!? What could the new Keyblade wielder & Xehanort's trusted cohort be talking about?"