If you're looking to build a team of villains to fight your heroes, you could certainly do worse than the Serpent Squad/Serpent Society. We don't even have half the full line-up yet, but they don't tend to send all their members on all their missions. Who have we got?
That's a start! Certainly enough to give Captain America or any solo hero of your choice a little bit of trouble before the major villains show up.