The Clown That Laughed
This Movie is the inspiration for DC Comic's "The Joker." I think it'd make a good riff and it's probably public domain due to age.
Anonymous? You do realize they riffed Casablanca, The Wizard of Oz, Jaws, Raiders of the Lost Ark.... I can't imagine they'll balk at the chance to riff at The Man who Laughs (1928). It can't be that much of a sacred cow.
Not sure how fast a turnaround Rifftrax and the fellas can get the rights (or a decent copy), write a commentary, and produce the project but this could be a great tie in for The Joker film coming out this October, 2019.
I think you mean "The Man Who Laughs." I doubt they'd do this one since it's so highly regarded as a legitimately good (and sad) film.