I like the sandbag idea, it would actually make a lot of sense to have at least some improvised fortifications in place when the base gets attacked... perhaps even randomly placed, so that the base defense missions have a random element?
as for sentry guns, this is actually possible, in a way. since items can spawn units (afaik), all you needed to create would be a unit which is incapable of movement and looks like a gun on a tripod (or something like that). you would be able to control it once it's placed, and the aliens would attack it as if it were one of your soldiers
somebody should start creating a mod for this... I don't know how to achieve the part about "incapable of moving, but still able to turn to shoot", but we'll figure something out.