Just a thought, but something that might be kinda interesting to implement would be something of a "friendship" meter for units that progressively increase over time as the soldiers fight together on numerous missions and have minor affects on things such as morale, accuracy, and possibly even affect certain actions that units could take in desperate situations. For example, if a certain soldier consistently is on missions with another soldier and tends to be within close proximity of each other during combat (say for example a shotgun wielder and a grenadier who provides cover with smoke or stuns during infiltrations), they would acquire a "friendship" bonus on future missions that they continue to partake in together. This bonus could be an increase in morale or a reduction in lost morale when within a certain range of each other. And inversely, morale loss could be increased when the soldier that they have a higher friendship with falls in combat (and this loss could be lingering for some time after the mission as well, especially if a soldier's friend were to fall in a mission that they did not partake in, as one would assume would be the case in any war where a friend is lost in battle).
If implemented, this system could even further advance the friendship concept by adding something of a free "inspiration" action that could occur when two units with high friendship perform a certain action within range of each other. Such things could be a coordinated attack option if two infiltrators are advancing simultaneously which would increase both units' accuracy and damage output when moving together, or even possibly inspire a free action if one of the soldier's friends is under fire and in a bad position such as covering fire to help their friend escape from a pinned position or even throwing a smoke/stun grenade at the attacker to inhibit their barrage. And a step further still, this friendship bonus could potentially be implemented in a way that when a soldier falls in combat while on a mission with a friendly unit, the fallen soldier has a higher chance to be revived after the mission is complete as if through some gushy bond of friendship that inspired the fallen soldier to give life one last chance so as not to leave his comrade behind.
Just food for thought, I'm sure there are many other ways this system could be implemented to further add to the realism affect of a war simulation game and add some more relatable human traits to the otherwise seemingly robotic soldiers.