Since it's an outdoor race, it's better that they pick the team during the outdoor season to see who is "on it" for the current year in outdoor settings.
But, if you had to pick a team right now, I'd say USA's Best 3 Rider team (1 small bore 250F and 2 big bore 450F riders) is:
250F - TOMAC (sure Roczen is leading him in points, but Tomac's the fastest 250F guy from USA, Roczen is not eligible for our team)
450F 1 - Villopoto - He's still the current "Man" in the 450F class, with the most wins and most consistent speed
450F 2 - Dungey - He's 1 small hair away from Villopoto, and clearly better than everyone else, plus he's the reigning outdoor champ
If those guys got hurt, who could we turn to as back ups:
250F backup - Baggett - Reigning outdoor champ, fast outdoors, not so good at SX
450F backup - Milsaps - Running great this season at #3 in SX and was #2 in SX last year
450F backup - Barcia - Running #4 in SX this season, won some races like ME Cup, and really wants to win badly, was our best guy at MXoN last year.
That's my take on the team as of now that I'd want to send to represent USA this year as of now.