A hero is as good as his or her opponent.
Given that as a usually-true statement, I am boggled that I haven't given more attention to enemies in my life.
My technique was that I would come up with situations, put the characters in them, and if any of the villains really resonated with the players, bring that villain back. So that was the case for Shrinker and the boys (I did give them a group name, but that was what we always called them), with Dark Silver, with the lovely plant-woman recently.
But I wonder if that was being a bit trusting.
Maybe what I should have done is (as per David Ducker's suggestion over on YouTube) is have every PC come with an arch foe. The player says why the foe is important; whether the character is designed by the player or by another player (as in the ICONS character creation system) doesn't seem as important to me, especially since villains evolve at a different rate than heroes do.
That buy-in from the player makes the arch foe important. (I've had players who weren't interested in the characters that I introduced as their "arch foes".)
Then, over the course of the campaign, you can introduce each arch foe. Essentially, the arch foe takes on the hero but has to deal with the other players as well. The arch foe loses (almost certainly) but then all of them can get together, join up to fight the heroes.
In a game like ICONS, which is forgiving of different power levels, that should work reasonably well.
I haven't tried this, because it really does call for a continuing series of adventures, and I don't have anything ongoing at the moment.
It could be an adventure series all by itself, a mini campaign: Four players, four sessions dealing with the arch foes individually, and then several sessions that deal with the combined League of Doom.