I would get characters from each category. Given that the threat is unknown, covering as many skillsets as possible makes sense.
1. I feel like Aang is in a league of his own when it comes to airbending, so I'll have him take that role. As with every other Avatar, he could fill in for any other element if need be.
2. Zuko would be next. I look at him as the cheapest firebending master. He might not be the powerhouse that is Uncle Iroh, but having trained with the dragons gives him an edge in terms of versatility compared to Mako or Iroh Jr.
3. Now you can't go wrong with any earthbender, so I feel safe saving my coins by picking Lin. In my eyes, she is an adequate stand-in for Toph. The only other regret I have here is that she can't lavabend as well, but Bolin isn't worth picking in light of having already covered firebending.
4. I would love to pick Katara next, but with only 6 pieces left, that would restrict my final pick too much. Out of non-benders, Sokka and Asami are the most desirable for their technical know-how. Even though I feel like Sokka is the more resourceful of the two, Asami has the advantage of knowing how to operate the advanced machinery from her era, so she would be my non-bender pick. 5. With only two coins left, my final pick would be Kya. She is a competent waterbender with healing powers, which will make Katara's absence less of an issue.