There's sort of a connected web of funny people who all appear in each other films, and Anchorman: The Legend Continues intends to make use of every single one of them. So it should come as no surprise that Adam McKay might have a little part put aside for John C. Reilly. Not only does Reilly have a proven track record with Will Farrell, but he's the funniest man alive.
Bleeding Cool published this rumor based on a casting call specifically asking for a double who looks like John C. Reilly. They're pretty certain this means John C. Reilly himself will be shooting. But they don't know that for sure. It might be for Joe Don Baker instead. Or Michael J. Pollard.
For now, we'll have to assume McKay intends to use John C. Reilly, though. It's doubtful his role will be much more than a cameo. But then, Adam McKay wants this movie to be absolutely filled with people, so every role's a cameo. Especially Michael J. Pollard's.
Supposedly Anchorman: The Legend Continues will come out this December, which is both really soon and really exciting.