-Dynamically scale speed with weight.
-Give mobility penalties based on weight borne. (Slower getting up and turning.)
-Add a "tired jogging" animation (I suggest using the "at ease" jogging animation the player character uses, it is slower than the normal jog, and looks a little hunched over, thus appropriate for such.)
-Make moving over terrain inclines increase fatigue (both up and downhill, uphill slightly more than downhill. Energy expediture can be roughly estimated by looking at how much power vehicles need to climb hills.)
This needs to be at least acknowledged, so we know someone in BI is looking at this.