Seriously, get "X-Wing Alliance" and play it. This game does extremely well in this matter. You get a mission, but eventually unwanted things occur and you're doing something completely different then. Although Il-2 and SoW are simulations, a similar way of building missions can make it more exciting.
Generally, I'd would really like to see a 100% dynamic campaign similar to the one EF2000 had. Everywhere is something happening, and you take part in it. While you select to fly mission 1 out of 15, the other 14 are done by AI simultanously. That was a great campaign! But I fear this might be too stressful for our hardware with the high simulation fidelty SoW will have. But still, the missions could be built to be anything but "08/15".
This could be: You mission is to intercept some Heinkels... now the weather changes quickly and they're designated another target where you can't intercept them anymore. Instead you'll fly just CAP then and encounter a single recon plane you have to shoot down.
Or: You fly a mission (doesn't matter what) and when you return to you base you see how some Stukas attack it...
Or: You have to attack things with the Bf-110... another group of 109s spots a small bomber group with escort fighters and they'll engage the fighters and call you to intercept the bombers, because you're not so far away. Drop your bombs in the Channel and go Blenheim hunting instead
In short: Build some missions where things happen that aren't in the briefing, sudden changes of the situation, weather changes, vulchers, order changes and so on.