I don't think anyone ever considered letting Imp/Reps do content together, cross-faction guilds would obviously be primarly about much needed improvement in communication in-game, the social aspect.
So how do they implement a cross faction guild and yet prevent guild members from grouping together?
I do not see how cross faction guilds could be implemented while still maintaining faction separation.
I think that it would be easier to implement "legacy friends lists" that function in a manner similar to the method that another company uses to allow cross server, cross faction and even cross game communication. This would mean individual communication, and not "guild chat", though.
Maybe allow the formation of cross faction channels? That way guilds could form their own "private channels" and guild members from either faction could join. This could effectively allow cross faction guild communication while maintaining faction separation in other aspects of the game.