Ok let's be honest...Declan
Dane aka Green Warlock's not that interesting yet. But I feel like we have enough clues to make an educated guess on who he might be.
We know the 2 places he's terrorized in the Scarlet Witch series are Greece and Ireland and that he's Irish. He's mentioned his mother and brothers. We know the family fought the Irish Gods the Tuatha DÃ© Danann and he's still bitter about what happened to his mother.
So here's my theory...Dane's mother is Carman from Irish mythology, the Athens born sorceress/warrior woman who tried to invade Ireland with her 3 sons Dub, Dother and Dian (Declan Dane?) and who was defeated by the Tuatha DÃ© Danann. Carman put a blight upon Ireland much like Declan has done in the recent Scarlet Witch issue.
Hmmm... Green? Dane? Makes me think of Polaris.
Seriously though, you might be onto something.