I was skeptical about this, but fans have brought up two actors who actually have the potential to play the role well. I believe Andrew-Lee Potts, who has actually played the Hatter before in a mini series, was brought up here. Also suggested around the web is Seth Gabel. Both actually have somewhat of a resemblance to Sebastian Stan, and who knows maybe one of them can play the role well in their own way. Both actors also appear to have a much more free schedule for such a commitment compared to Stan.
Here are photos of the two actors, one with Andrew in Mad Hatter attire from “Hatter”: