A bit of a late response, though I found that modifying the number of units in a mission can make it more interesting and/or possibly very difficult to complete.
While not the same as relative random target acquisition/encounters,
Say a mission calls for 1-2, 2-3, 0-1 (random units in mission) editing to say 2--3, 4-6, 1-3 and corresponding Defensive/Offensive role, makes seemingly easy missions more interesting as you're not sure what you'll encounter or how many.
Just my .02.
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