What shows *should* be part of the X-files universe?

Apart from Millennium and The Lone Gunmen, what movies or TV shows should/could be part of the X-Files universe that would add to it without major disruptions?

Resident Alien would fit easily (IMO)…but are there any less obvious answers? Maybe Seinfeld (even putting aside Kramer as a monster of the week, there were a LOT of unexplained coincidences that could attract Mulder on a slow day)?