Make Captain America great again!
Captain America is one of the greatest Marvel heroes but the classic version sucks in this game and WW2 isn't far behind.
Holding block should allow him to block attacks that are unblockable.
It's simple, not overpowering or game breaking, won't require completely reworking the character and would make him a more useful and desired champion. It could be further expanded to include being able to block heavy attacks with his signature ability, that could be scaled to a percentage of heavy attacks or to once every so many seconds if that would potentially make him too overpowered.
In comics and on film Cap blocks so many powerful attacks with his shield that have destroyed whatever they previously hit so it would fit the character that he would be able to block unblockable attacks in the battle realm.
For the record I am not suggesting he has perfect block and would take zero damage from all attacks, he would still recieve block damage.