Or Tahno and his sexy hair.

^ :D Wow yeah Tahno would be interesting to see again , especially after a 3 year time skip
And i personally really wanted to see Tarrlok and Amon >_< I mean they started redeeming Tarrlok why didn't they finish it , maybe show how he or someone else from the 1st season whom had lost his bending developed and didin't give up despite the big loss or something like that and how he didn't take bending from something given anymore , I honestly wanted that to happen to either Tarrlok or Tahno or even Korra if she hadn't gotten it back so fast.
And Amon was a super fighter and it would have been epic see him battle Spirits or someone from the Red Lotus >_<

Didn't those two die? I don't know, I think there was just so much they could have done with all of their villains. I wanted more of them.

All of them actually. They were all really entertaining, and I preferred watching them instead of Korra.

Tahno <3333
Also Desna & Eska. They did make an appearance in Episode 3, but I hope they get a bigger role this season.

Amon.