Nick Fury fights a Super Skrull, IIRC. Maybe not a Thanos-level big bad, but a big bad.
Oh, I know...I'm just not of the opinion that Captain America has to be juiced. That's it. It's not like Steve gets into every battle because he has the serum; he gets into it because he believes it's the...
Wait, aren't there a lot of MCU characters who don't have superhuman or supernatural powers? Black Widow and Hawkeye, like you mentioned, but Tony Stark, Nick Fury, Peggy Carter, Bucky (not 100% on this one), Daredevil...