This is indeed creative, but I'm afraid that this idea as it is doesn't have a high chance of making it into the game.
We already have plans for UGVs which will possibly do some of these things, although what stands out to me as a problem is:
1 - The idea of complicating the game with new mission types just for a specific new weapon/tool,
I don't think adding more options is complicating the game. One could still do missions without using gun emplacements at all. I often see the tendency do simplify some good game principals. That is the main reason why some of the old stuff is still prefered above the graphical superior new stuff. Look at the discussion started as civilization 3 was released, how mad people got, because many of the possibilities civilization 2 had have been taken out to simplify stuff.
I don't say games should get too complicated, that succeeding in a game needs to study hard in game mechanics. But I don't think, one should fear too much in a game getting too complex, as long as the game mechanics still feel natural.
The idea a gun emplacement may add a civilian exit point is a bonus for this idea. I still would love to see a gun emplacement in this game without these exit points. I just thought, if a player takes time to do some fortification, he might as well be rewarded for that.
and,
2 - The idea of lots of blasting of inaccurate "cover fire." It has the same issues as air strikes and bombs. Phalanx is supposed to be a secret, discrete organization, low-profile. The player's units are more supposed to go in and cleanly pick and choose targets while saving civilians, not just blow everything up in a barrage of massive cannon fire.
Edit: I also had a grandfather who fought in two world wars - He once told me that many people favored the idea of tons of blasting everything in an area to total destruction - a popular concept, but even in conventional warfare such a thing has been proven to generally be inefficient in defeating an enemy, and it isn't the best option for fighting.
I thought to have read a discussion about phalanx be not a secret operation, but a operation with full nato support. There was a discussion about, if phalanx should be a secret department, like xcom was, and I think it was eigher you, or mattyn, who said, it is not on purpose.
However, i understand phalanx has no desire to blow up areas big, because this could cause bad news. Phalanx is, however, a new facility, that could also be dismantled due to too much collatteral damage. But looking at the original xcom series that organisation didn t mind to use explosives, explosive ammunition and such at all. Ever played xcom apocalypse with autocannon explosive shells on autofire? :-P
The idea of cover fire is not to actualy hit anything but more in lowering the enemy morale. One in the area of cover fire might not be willing to raise his head to watch, if someone is approaching. I already wrote i intend the cover fire to be at very low accurancy. So one using it a lot will see, he is vasting much of ammunition without doing much damage.
However, if there is ever a feature added, that someone will have lower accurancy, if it was shot on him (even without being hit), coverfire may get useful, even without hitting anything. The lower accurancy would be justified, imho, because one has to fight his instinct to take cover, to be able to shoot.