This type of mission would require a lot of scripting, but if done correctly, it could make for a very sweet mission and it would have lots of replayability.
Here's the basic plot. You are an officer of a 12 man platoon at your base, well behind the front lines. You receive a radio message that one of the towns on the frontline is being attacked by the enemy and you need to go reinforce it. Simple enough. You order your men to get into a truck and start to head down the road towards the town. During your trip, you get another radio message from another town saying that they are also under attack and need backup. Very shortly after that, you get a message from a third town, proclaiming that their recon teams have picked up a lot of enemy movement in the area and believe they are going to be attacked soon. They too need some reinforcements. Now you have to decide where you want to take your men and where you pick will determine what happens throughout the rest of the mission.
For example:
If you pick Town 1 - You arrive at town just as they have destroyed the first wave of enemies and the commander is glad to see you, saying that they lost a lot of men in the first assault and need everyone they can get. You get your men into position and prepare for a 2nd wave of attack. HQ has received word of the damage and have sent reinforcements as well who arrive either in more truckloads of soldiers or from choppers. Either way, you get more men. The next attack comes in and you have to fight them off. There could even be a third attack if you really want to keep the player busy.
If you pick Town 2 - You arrive during the battle. You could have it so that after several members of your team get out of the truck, a tank blows it up killing some of the men who were still inside and reducing your number. That might be hard to do without killing the player, but it would be interesting. Anyways, you have to get out and disbatch your men very quickly and attempt to fight off the enemy. You will get a few reinforcements during the battle but not many. This town will actually be outnumbered and after several minutes of harsh battle the commander of the town will determine that all hope is lost and order a retreat. The retreat can either go to one of the other towns engaged in battle (Town 1 or Town 3) or it can end the mission as you head back to your main base.
If you pick Town 3 - You get there right before the attack takes place. This town is close to an enemy town/base sitting right on the border, but far enough that it's not visable from the town. This town is heavily equipped with tanks, machineguns, soldiers, you name it they have it. The enemy attack is fairly weak and pretty easy to disbatch. After a small fight your town commander declares that the defense was a success. Several soldiers in the town are still pumped up after the battle and want to go after the enemy town that is close by. The commander reluctantly agrees and orders several platoons, including yours to advance into enemy territory and attempt to capture the town/base near by. You could make the base heavily guarded so that it's a very tough fight... or you could make it so that the enemy scout team detects that you are moving in towards the base and the enemies disbatch several soldiers/tanks/choppers to intercept you. If you survive the ambush then the town/base will be lightly defended and fairly easy to capture.
Also, to add to replayability, you could have it set so that different things happen in the towns that you do not go to. If it's possible, having random things that could happen to each town that you don't go to would make it even better. For example, if I choose to go help Town 3 one time, Town 2 will win their battle and send additional troops to our town to aid in the attack against the enemy town. But if I play the mission again, and go to Town 3 again, maybe Town 2 will lose their battle this time and we won't get any additional troops from them because they are retreating to Town 1.
Like I said, it would take A LOT of scripting, but man, that would be a sweet mission huh?