Several of these guys feel like they've never really gotten their due.
Blastaar has come close, but could go further, leading an invasion from the Negative Zone (through the portal in Four Freedoms Plaza, now the center of his kingdom on Earth, centered around a devastated New York City).
Attuma is another with potential armies, who never seems to get the gold. Namor has invaded the surface world, it seems, more than his 'villain.'
I always loved Diablo's mystical chops. And he's shown some insane potential. Turning an entire town into Hulks as a distraction? Yikes!
Both the Red Ghost and the Leader (along with the Wizard) *should* have gotten the 'evil genius mastermind' roles that went to comparative dullards like Norman Osborne and Otto Octavius. (Although Otto has at least had some brilliant stories come out of it!)
Umar has too often been 'Dormammu's sister,' and her own stories kind of subordinate to family politics. She's either supporting her brother, replacing her brother, or being replaced in turn by her daughter, but since Dormammu *always* comes back, and *always* ends up in charge again, she seriously needs to find her own 'Dark Dimension' to rule, rather than temporarily babysitting someone else's throne while he's away.
And the Basilisk. I wait and wait for the Omega Stone to turn into something to do with the Kree and Skrulls and their various cosmic schemes, and for him to get dragged into the Marvel Cosmic side of things, alongside Earth folk like Richard Ryder/Nova and Heather Douglas/Moondragon and Frankie Raye/Supernova. But no. Squandered potential!