Previously, many of us have introduced uncertainty into solo wargaming by having enemy units arrive each round at a randomly selected edge of the battle field. This works well and I heartily recommend it.
However, you can take the idea further when playing modern forces. Consider airborne units dropping at random locations on the battle field. This could apply in World War II scenarios, modern warfare, or future warfare. Science fiction scenarios might have troops arriving in space shuttles or even as individuals (as Iron Man does).
When airborne units arrive, they might be scattered and will need to regroup. Their battle plan is to cluster their forces as soon as possible. As the defender, your job is decimate units as they arrive. However, you would not engage 1-1. Instead, 2 or 3-1 would be better. Lacking those odds, you would withdraw and attempt to link up with your other units.
Your tactical challenge is to use cover well, to prevent the enemy from grouping up, and to avoid losing battles. And, of course, to be lucky when rolling the dice.