I agree with this theory. I feel that Li Mei could possibly be Bo' Rai Cho's next student. But what could BRC see that Li Mei only has? I think there's something personal goin gon here. Not between Bo' Rai Cho and Li Mei (unless they're father and daughter), but between Li Mei and someone on The Dragon King's forces. Maybe Li Mei is somehow related to a bad guy, or she gives up joining Onaga because she wants to fight in what is right, not what will get her a free pass.
Or there's the Joan of Arc idea.
BTW, I think the armour she wears is something like the national Outworld armour sort of thing. Onaga, Sub-Zero, Li Mei and the Dragon King's forces wear it.