I think the turret idea is a really good plan but had a few ideas on balance. For instance, working with the assumption that optical recognition tech is much better by the time the aliens invade, but not so much it is cheap (still processor intesive) we could give the turret three fire modes that would determine how many "reactions" that it gets. Motion mode would fire at anything in its arc of fire (you, civis, them) pretty fast reaction, lots of shots, IFF mode (not me) slightly less shots, slower reaction, and optical mode (only aliens) slowest reactions, one or two shots. That would really level out the use of the turret so that it was useful, but not gameplay defeating. Adding a limited arc of fire and fuzzy accuracy (ecm from aliens disrupts electronic systems) and I think this would be workable. I remember that scene in aliens in the maintance tunnel and can't help but thing how cool a pair of those set up would be.
Some of the other techs I was wanting to see were riot shields. These could go into the offhand for 1hand operation of pistols and the like, this way your troops would have high armor from the front and adding a new dimension of tactial play (think shield wall)
About the aircraft cannons, I'd suggested in the aircraft thread recently that the pilot should strafe the LZ if the dropship is armed and dead aliens could be found around it based on his/her skill, would that suffice for the dropship gun? They would also be able to see/spot some of the aliens so you had round one LOS, thus taking care of some battlefield intel. This could all be skill based for more depth.
As far as other techs, we need more gadgets for combat. Things like motion sensors, prox mines, alternate ordinace (gas/plasma/antimatter rockets/launcher grenades, incendary munitions for human weapons, remote cameras, flares, or some sort of spider grenade/mine would be cool. I can remember the brainsuckers from apoc being pretty cool when they'd activate and chase your team so this would be a nice addon for both sides.
Just a few thoughts on what I noticed was missing,
Guildenstern