I don't see much behind Yuffie's teasing with Leon (Squall is dead to him at this point). I see their interaction as a more friend base. But who knows, they didn't really give Yuffie much camera time, especially interacting with Leon. It's a possibility!

Plus I'm a Squall/Leon/Rinoa fan. I would be happy if she came into KHII just to put him back into shape.