Re: James Buchanan Barnes aka The Winter Solider
After reading that the Invaders might be featured I had an idea how they could introduce Bucky:
Perhaps Steve puts together, or is assigned to lead, a special forces team. Sort of a specialty squad of like the Howling Commandos.
Soldier Bucky Barnes could be one of those soldiers assigned to Cap's team. They don't wear costumes exactly, but maybe modified and special uniforms.
Bucky could end up working well with Cap and become is second in command, and a partner.