Re: Plot for Mastermind
Sounds like a version of Vandal Savage. I'd say have him take the long view on virtually all his plans and only dabble in local and immediate villainy to alleviate his boredom. Have him be (or become) a highly respected philanthropist who gives regularly to charities, maybe even sponsors a superteam. Start dropping clues every other adventure or so that maybe he's not who everyone thinks he is. Maybe his "long term goals" is to simply not be board, and he just messes with people for kicks.
For example, maybe he could sponsor your PC's team--give them cool headquarters, tech, etc, with no strings attached. A few adventures later have the HQ broken into, or attacked. Maybe one of the heroes has his secret identity almost compromised by a villain, or maybe a villain (or team) simply know much more about the heroes than they should.
Slowly reveal the fact that the Mastermind helped the supervillains the same way he helped the superheroes, and kinda nudged them at each other to see how they would react.
Go for subtle, not the typical "Mastermind vs. the PCs" plot.