That idea is old and quite used. It has been done as public MP co-op maps, and especially as training grounds for co-op clans in various styles and versions. I personally use just such a mission for Project: Flashpoint (see sig, or read OFPEC news). Basically what you can do to make it more interesting is to take it in two directions, preferably both:
1) Background, background, background. 30 minutes just to explain to every troop how you are doing it, why you are doing it, and split people to different flanks. Just getting in position should take more than an hour, at least. Every precaution must be taken and the plan must be water-proof, even if it's "just" training. This will make the players feel that they are doing something that actually matters. Roleplaying is extremely welcome here.
2) Flexibility and chance. Assaulting the town at the border-line can get kind of old the eleventh time you do it. Randomize
every bloody unit. Put a convoy on the map, make it move randomly. Put out a few hot-spots, like guarded officers, fuel dumps, harbours. You don't have to take them every time you play, but just have them there. Mix in a bit of roleplaying and a really good groupLink/reinforcements/support/whatever and you can basically have every bloody scenario you can think of.
You CAN take even one step further. I am personally working on a MP-mission which will let me randomize everything, from patrols to convoys and objectives - without me even interfering - AND it is communicating with previous runs of the same map. What this means is that if you lose that vehicle, it's lost. But on the other hand, if you take that town - it's yours and will stay that way no matter how often you reassign. At least until you're counter-attacked.