I have been using the “Threat Generation System”, written by Martin England and published in Miniature Wargames 373 to use as a solitaire “opponent” in my recent WW2 games: I have reviewed the system here: http://hereticalgaming.blogspot.co.uk/2015/12/review-of-martin-englands-threat.html perhaps other solo wargamers may be inspired to get a copy of the article or at least examine some of the ideas to see if they are useful.
If you have read my recent Op Martlet AARs or WRG review, no need to click on the link for examples within the review, as it points to the same battle reports.
One thing I should add is that the system should support co-operative play as well as solitaire play.
I came up with something similar for my solo Soviets in Afghanistan campaign some years ago.
The campaign is based on a tour by a Motor Rifle Regiment and most of the actions revolved around Soviet sweeps or convoy movement. The Mujahideen needed something so I didn’t know what was where or how many enemy I, as the Soviet commander, was facing. So I cam up with an idea for the max/min forces I was likely to face based on my (somewhat shaky) int and then mixed the minimum number of chits with equal number of dummies to place on table plus a number of blanks drawn from a mix of the max number of HQs and equal number of dummies. This gives uncertainty about the number of enemy units and their composition within realistic (I hope) parameters.
The actions were pretty clear usually but ad hoc dicing for aggressive or passive postures were thrown in as required.