Empathy
When you want to make a mission that will engage players and allow them to creatively solve problems, you should generally avoid thinking linearly. The first step in order to do this is trying to get into the other faction's mind, and ask yourself: "how would I do this?"
In general design, no matter the field, empathy is an important aspect of it. Thinking about the players, or about the enemy faction, is a great way to ensure a mission's design is consistent and realistic.
Imagine you set up a group of OPFOR to defend a location but this location has many entrances, aside from the front door, there's gaps in the wall, there's a back entrance, maybe there's a forest nearby. These are all things that a competent enemy would think about when setting up an efficient defense for a position, unless you purposely want to depict the enemy force as incompetent.
In this case, you can add a patrol outside and around the compound. Maybe a guy or two guarding the gaps. This will account for different ways players could possibly take on this objective, instead of just one.
Try to get in the enemy faction's mind and think to yourself how would you utilise the resources you have to complete a task, whether it is an attack, an ambush, or the defense of an important strategical keypoint.
It is nearly impossible to always know all of the ways players could possibly take on an objective, let alone how a mission is going to play out. Accounting for this kind of variability is a key sign to a thoughtfully made mission.