I have used lots of sources, from the map itself, to my imagination, to fiction, historical records, and news reports. The SecOps manager is great for really fast, on-the-fly missions that take next to no time to make.
My latest mission is a high command mission where the player controls an understrength Russian spetsnaz/airborne company in a blocking position, based on a Soviet battle in Afghanistan and a few fights in Chechnya.
The thing I most try to do is to make things as replayable as I can, as I tend to play my own missions. If my own enemy that I place can surprise me, I know that I have done well.