Not sure if you're talking about dressage or eventing, so I'll do both.
In dressage, Brazil should be the favorite for the second spot, but their level is dropping, so Mexico and Argentina should definitely have a chance for a surprise.
In eventing it very much depends on how many teams qualify here. It's quite possible that none of the American teams will make it (USA has been anything but consistent over XC in recent years, even though they'll be very determined this time), in that case it will be USA and CAN as the favorites. If USA makes it, then it will be hard to beat CAN and BRA. Finally, if two teams make it here, then Ecuador will have a very good chance.