Put interesting events and situations in your missions.
I have one, a SEAL team landing, that has destroying an enemy training and special operations complex as an example.
On the way there, they pass a small, walled re-fueling station. Since this is a rear area, the Soviets are not trying to be quiet or stealthy, they are just doing their normal jobs. At this gas station, a T-72 tank has broken down just outside the station. There are red flares around it, a repair truck is illuminating it with it's headlights (It's 5 am), a sentry is chatting with the driver of the repair truck, and I used a snippet to make a soldier, acting as a mechanic, drop his AK near some barrels, then sit at the back of the tank. I cycle his actions, so he moves a bit (tries to stand for a split second) then sits again. He never does stand, but he moves a bit while sitting, as if he's working on a tread of the tank. there's radio playing music next to him, and just like the flares, I used a runway marker, this time in white, nearby that seems like a flashlight. I timed the SEALS approach to the gas staion so that a BMP shows up to try and get gas. They stop near the tank, get out, and stand by the gaurd who's chatting with the repair truck driver. It really looks like they're shooting the bull.
The player shouldn't even attack these guys. It's just atmosphere. But it gives them a location to avoid on ex-filtration, and it gives an illusion that the bad guys don't all sleep at night, they are out on patrols, assigned to mundane tasks, and even slack off now and then.
For me, a few random events like this can make a regular mission at the least interesting, and at most, put a twist on it.