(I won't go ahead and say Hibari, because fandom would kill me... but. Well)

LOL.
Well, i would really love to see Haru replacing Lambo (sorry, Lambo fans). But then Chrome's position as the smurfette will be threatened.
If Hibari is not the guardian, he can be one of Vongola's opponent. The idea intrigues me... imagine the ensuing angst for Yamamoto and Ryouhei *A*