In my opinion, I believe that Lea should be a party member for Kairi's journey wielding his keybade, which currently has an unknown name. However, I do believe that in Kingdom Hearts III Lea should be playable through versus mode. I think that unlike KHBBS, Versus mode should have many playable characters including Lea and other characters from the game. To find more about my opinion on a KHIII Versus mode visit my upcoming blog bost: KHIII Versus mode.
Since Lea is a party member, like Donald and Goofy he should have more than one keyblade to use:
- Lea's Keyblade( his unnamed keyblade from KH3D Dream Drop Distance)
- Frolic Flame
- Hidden Dragon
- Bond of Flame
- Heretic Flare
- Twilight Blaze
And hopefully new keyblades made specifically for Lea
As a party member he should mainly do fire-based attacks and as a limit command he and Kairi together do fire-based attacks using their keyblades and end with a huge explosion.
He and Kairi go on a quest to find Master Aqua by finding a hidden portal to the Realm of Darkness. One of their final bosses should be Isa, since Lea is the party member. The battle will most likely take place during the Keyblade Wars leading up to Sora, Kairi and Riku's final battle with Master Xehanort, weilding the x-blade.