Just throwing out an idea.
Normal jumps and super jumps both affect the camera in different ways. A normal jump will pull the camera to about the lower torso level of most characters, but super jumps pull it well over their head, making them unseen.
Cap's double jump has a seriously fast fall speed. Super jump + double jump at min height = assist high/low shenanigans?
Call a low hitting assist like Deadpool's Quick Work or Wesker's Samurai Edge and do this, but come down with quick L's or something else? I think you have a seriously bootleg Cap unblockable with proper timing. Cap falls extremely fast so you try to react to the air move, but the low assist pretty much isn't seen. The only thing I'd wonder about is locking them down long enough to call the assist and jump. Even then, maybe something like Anti-Ground kick would just blow up an attempt to stop you coming down. Or you just get super'd + happy birthday'd for your attempts at trying to make Cap go above mid-tier.
Obviously the super jump isn't necessary, but not being able to see the assist itself plus Cap coming down with javelin speed could confuse you.
Haven't tested out the whole "unblockable" thing, but the super jump mixups have served me well for now.
Thoughts? Have I found something new? Is this old shit? Is this the ramblings of a Cap player gone desperate trying to make him better?