TacSim is NOT just going to be generic challenges.
They would not sell cosmetics exclusively for a mode that was just:
-shoot/takedown 10 enemies and get a HIGH SCORE!!
That's way to generic for what appears to be a major part of the game as well as the confirmed post-launch content.
I believe TacSim will look like this:
-Generic Gameplay challenges for a quick hit. (30-40%)
-Sandbox-Section-Replay (and maybe linear action sequence replay?) with additional challenges (40-50%)
-New Missions on existing maps/new maps(probably not at launch) (10-30%)