There is a story floating around that says that Torchwood may get more international for series 4. Apparently, a source on the Outpost Skaro message board claims that the BBC is looking for more of an international flavor to the series - especially America - and that:
“Cashing in on the series popularity stateside, one of the suggested ideas is a twisty turny time travel tale which will include the appearance of a few sci-fi luminaries including Nathan Fillion as another Time Agent, Brent Spiner as ‘a senator’ and the return of James Marsters as Captain John.”
The source also mentions that they are only at the discussion phase.
Please, take this story with a HUGE grain of salt. I personnaly think this is just wild speculations and a fan's personnal sci-fi wet dream. However, stranger things have happened so we'll see. Stay tuned.