I see the Masters of Evil, possibly only going by the name "the Masters", being somewhat like the Ultimates storyline where they fought a team of "heroes" from outside of America. I could see this team pulling itself together thinking they are their land's lines of defense against the American Avengers. The team could have their "heroes" of those nations and states. And backed by their soldiers like SHIELD backed the Avengers.
That could be a way to go.
Another way to go would be that their creation is somehow connected to the rise or return of Hydra, led by Baron Zemo
and Baron Strucker
. It could have Zemo leading them, Crossbones
as his enforcer/assassin along with Sin
the genetic daughter of Red Skull (using the Skull's DNA to create a daughter for him after he vanished) as another assassin. And then others on the team could have heavy ties to Hydra. With also Enchantress
(there to trouble Thor and his Earthly friends who keep him away from Enchantress). Make it all very terrorist related and not some big evil superheroes thing. More like a terrorist funded team to take down the terrorist's biggest obstacle. There could be lots of Hydra soldiers and lots of SHIELD soldiers as well.
Thats another way to go.
Make them heavily involved with the Terrorist stuff of Hydra. Makes Hydra more dangerous too. And if the Avengers continued like in my idea to rotate and stagger the cast, eventually they could bring in characters like Taskmaster, Grim Reaper, and Wonder Man.
How would you want them to fit into the Marvel movie world?