Ford last stuck the 5.0 badge on the Mustang when it was last powered by a 5.0L V8. So now that the 5.0L Coyote V8 is expected to replace the current 4.6L V8, it makes sense that the 5.0 badge would return.
That logic is backed up by reports from a post at MustangSource who claims a Ford parts list contains an entry for the 2011 Mustang with the 5.0 fender badge.
Expect a badge familiar to fans and highly reminiscent of the badge used from 1979-1993 - chrome numbers with a red 'dot' between the 5 and the 0. But also expect it to get a modernized tweak, much like the running horse badge did for the 2010 Mustang.