All have been batted around as possibilities, but today Deadline is reporting that Nicolas Cage, Wesley Snipes, Milla Jovovich and Jackie Chan are in "advanced negotiations," according to Lionsgate CEO Jon Feltheimer, who is a bit more reliable on the subject than Sylvester Stallone's late-night tweets, which sometimes seem calculated to get a reaction (he recently mentioned Mel Gibson, for instance).
Cage would presumably be the villain; Chan, of course, never is. Jovovich is an interesting choice in that she's still actively fronting her successful action franchise, in contrast to the series' usual practice of getting together guys past their peak. Maybe she just looked at the existing cast and thought they were zombies who needed slaying. Snipes is so obvious I can't imagine he'll bow out.
All four will have their acting muscles tested as never before, as they will have to be able to convincingly pretend Stallone is the most awesome guy in the cast. Think they're up to it?