I NEED it. I need that EXACT look on his face xD but hopefully he won't try to arrest anyone for unlawful conduct of giving Avatar Korra the 'eye'. 

I think them seeing each other might depend on how they get that youth-giving serum. Another thing we might be bouncing back and forth with until we find the right moment for it to come to life. I know he'll be stunned. Tenzin will probably look like Aang does during the series.