Thomas Haden Church filmed a mystery role in a superhero movie in 2017 - but the question is, which one? Given that Church said he's appearing in a non-Marvel movie, which we'll take to mean that he won't be appearing in non-MCU movies as well (from either Sony Pictures or 20th Century Fox), then that really only leaves three options (assuming he means live-action films, of course): Aquaman, Hellboy, and Glass - all three of which are superhero-centric films that entered production in 2017.
James Wan's Aquaman, starring Jason Momoa as Arthur Curry, is the biggest non-Marvel superhero film releasing in 2018. In fact, it's the only DCEU movie that's releasing this year due to constant delays for Flashpoint (which was originally slated to debut in March 2018). Aquaman wrapped up filming in October 2017, so it's technically possible that Church could appear in the film. But considering that Aquaman's cast is already filled out and the production has been heavily publicized thus far, Church's involvement with Aquaman doesn't seem likely, especially since there would've at least been a rumor about his role by this point. Still, anything's possible. And if he does appear in Aquaman, he could potentially be Xebel, since the underwater nation does play a part in the live-action film.
Another option is Neil Marshall's Hellboy. The upcoming R-rated reboot stars David Harbour as the eponymous character, and it entered production this past September. While the project made headlines when it was first announced as well as when it started filming, the actual production has flown under the radar (barring a controversial Ben Daimio casting choice), so it's entirely possible that Church could be appearing in that film instead of the DCEU. Since this will be an origin story of sorts, it would be interesting if Church played Hellboy's father, Azzael, or his uncle, Astaroth. After all, both characters play a role, albeit small, in the Blood Queen's story.
The third option is M. Night Shyamalan's Glass. It's not only a sequel to 2017's Split but it's also the long-awaited follow-up to 2000's Unbreakable, which makes it the third and final installment in Shyamalan's unofficially titled Eastrail 177 Trilogy. Even though Split was a psychological thriller, Unbreakable was a superhero film, and Glass is expected to be another superhero thriller. And since Glass wrapped production in December, that makes the sequel another possible candidate for Church. Taking into account that Glass is an original movie, though, it's unclear who Church could be playing, but his addition to the already stellar cast would be warmly welcome by Unbreakable/Split fans.
Of course, Church's comments about not appearing in a Marvel film could only be referring to the Marvel Cinematic Universe. If that's the case, then there are several possibilities for him, including Ruben Fleischer's Venom, starring Tom Hardy. It would certainly be interesting if Church were to reprise his Spider-Man 3 role as the Sandman for Sony's Spidey spinoff. That would make Sam Raimi's Spider-Man trilogy canon in their new villain-centric Spidey universe. But we'll just have to wait and see what happens.