If you are going to make a long campaign like that you will need themes throughout it, as well as the main plot. For example, as the front line ebbs and flows you may have to take the same village several times: each time from a different direction, with different forces, against different enemy, at a different time of day, with different weather, different supporting arms etc.
Another example, when the campaign is finished it turns out that the player will have driven every inch of road on the island .... but none of it more than 3 times.
Don't break your back trying to think up brand new ideas, there ain't no such animal. If you do think you're getting a bit stale, read some decent military histories and biographies and you will find all the scenarios you could ever want.