Just as the subject says, I think this would be an interesting mechanic to really make you feel like a leader. Effectively, it would work like a backstab of sorts, but for your minions. But only if the flanked enemy is being actively targeted by the two characters that are flanking (this includes you and your minions). On top of that, there could a bit of variation on the bonuses given while flanking depending on how flanked the enemy is. For example, if one minion is in front of the enemy, while another is to the side of the enemy, that would be a minor flank, giving small bonuses. While one minion being directly in front of the enemy, while another minion is directly behind the enemy, this would be the maximum bonus. This would make planning and micro managing your minions to separate the good from the great.
Some types of minions could even specialize in flanking. While others would be difficult to flank, but more of a payoff if they do. Maybe even have certain abilities have additional effects if it is used against a flanked enemy.
I bring up this mechanic because in CoX I would always set up my minions while my group would prepare. But in the end, the set up amounted to nothing. No matter what minor advantages I got from the setup, it was over all not worth the effort.