There's a couple of the Saintsbook challenges that require you to have STAG forces available to kill. After you complete the game, however, the STAG forces are nowhere to be found. I believe that you can fight a few more STAG forces if you replay the final mission, but I think my save is glitched, so that's not an option.
Is there any way post-game completion to bring STAG back? I miss their much nicer tanks and VTOLs for hijacking :(