Actor Dean Haglund, best known as Langly from The X-Files (The Lone Gunmen) revealed he is in Vancouver shooting an episode of the eleventh season. As mentioned in his podcast, he won't appear as a vision or an alien, but as Langly himself.
At this point, there aren't any details about his role, episode or the other Gunmen's actors.
Here comes the confusion (again). In the show's ninth season The Lone Gunmen died in 2002, sacrificing themselves to stop a bio-terrorist. However, in the comic book series The X-Files: Season 10 it is revealed that the Lone Gunmen are alive and well, having faked their deaths during the events of Jump the Shark (Season 9). The group was aided by the FBI and have since been working underground, aiding the US government.
This comic book was presented as the continuation of the series and the official canon. After the announcement of the show's return, the comics (Season 10, Season 11 and ongoing title, simply called The X Files) have been considering as non-canon.
In the episode Babylon (Season 10), they're considered to be products as part of Mulder's hallucination.
The full episode from his own podcast can be found here: Chillpack Hollywood Hour.
Also, you can listen to Haglund's chat with the owner of Hellblazerbiz podcast, dedicated to TV shows, movies and games. Once again, he confirmed he will return as Langly, but without giving out any details.