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The general selection pattern with Amazing Yamaguchi thus far is hero and his counterpart villain, who would it be for Cap? Well more like to have a good ratio of heroes and villains they pick someone who has some connections with the heroes that were made. But CA has some popular villains.
Red Skull could look cool and distinct with Yamaguchi's sculpt for long coat characters.
But Winter Soldier is definitely more popular now, plus can serve as both a bad guy or a partner to Cap. And he will continue to be in MCU, likely the next CA so there will be a long window to capitalize on his popularity. Yamaguchi is also good with the military type characters.