Comics giant Mark Millar is quite busy these days, both with writing new work and adapting his previous work for Netflix. If anyone is hoping that he will return to either Marvel or DC, they may be out of luck.
Millar has contributed to both comic giants in the past, including Superman: Red Son for DC, and The Ultimates, Civil War, and Old Man Logan for Marvel. He buried any thoughts of him returning to either publisher with a single tweet: After a fan asked him if he would ever work for either of them again, he responded by tweeting, "I'm full-time on staff creating TV shows, movies, and comics for Netflix and loving every pico-second. No plans to ever do DC or Marvel again!"
He's not saying "at the moment," either... the "again" in his tweet makes it seem pretty final. Even if he wanted to return, it's doubtful that he would have the time. As he mentions in the tweet, he is hard at work on Netflix adaptations, as Netflix bought the rights to all "Millarworld" properties last year.
His work in comics hasn't slowed either, as he's still working on his "magic meets the mob" series The Magic Order, as well as a new Kingsman comic, a new Kick-Ass comic, and a possible Hit-Girl spin-off title.
While there is no word on whether his new run of Kick-Ass comics will receive a live-action adaptation (Kick-Ass 2 was far from a hit with critics), Millar has already said that he wants Westworld/Thor star Tessa Thompson to play the new character of Patience Lee, and Thompson herself has said that she is "highly interested." The concept is still up in the air, but Millar did just partner with a streaming giant that has a hunger for original content... so there's that.
All of this is to say that there is a lot of good things coming from Mark Millar — just don't expect him to drop them all to go back to DC or Marvel.