I was thinking Damien as Mako and maybe Rick as Bolin (although Lex would also fit that part pretty well).
Honestly I was kind of hoping to see Rick play Tenzin, but that may be a bit too obvious.
Definitely Mac for Korra though.
I remember thinking about firebending possibly being roll-less back with Aang's and Zuko's lessons, but I had basically forgotten about that until the dragons showed up.