There is literally no franchise Joss Whedon would not say yes to right now. Whether he should be asked is a debate for studio boardrooms and comment sections on the Internet.
In an interview with Collider, Whedon talks about the script he’s working on now (“a historical fiction slash horror movie about a time when the world was going insane, World War II.”), Wonder Woman, and how Marvel and DC are doing in the live-action space. But he also answered the “Would you do another franchise?” question by basically saying he was up for anything:
I mean, it’s a fun thing to do, to put yourself in the service of something if you think you can add an interpretation. It’s no different than any other storytelling. There are some times when you get micro-managed to death but with Marvel, they let me make two movies that were very much mine. So do I want to make James Bond movie? Yeah. Anne Hathaway does Catwoman again? Sure, I’m in. Do I want to make a Star Wars movie? Yeah. I was like, “I don’t want to make a Star Wars movie. Like, god dammit, why?” But I saw the trailer for Rogue [sic] awhile ago and I was like, “I want to do that.” To make a Star Wars movie and not be wed to the bigger picture.
This is a much more laid back answer than Whedon would have given a couple years ago. And also a much more open one than most directors give. He didn’t hem or haw, he just said he’s good with whatever work people want to throw his way.
However, he may be a little bit off on his assessment that Rogue One is “a Star Wars movie” that is not “wed to the bigger picture.” It’s literally a prequel about the events that start Episode IV. I mean, I guess none of the major characters show up, but it’s still part of a larger story. It’s not that standalone.
In fact, the new Star Wars canon is much more connected than the old one was. And the thing people seem most excited to see in Rogue One? Darth Vader.
Oh well, maybe there will be a true standalone movie by the time Disney and Lucasfilm get around to giving one to Whedon.