I don't think it would be encroaching on Xenoanuts at all, merely fixing a problem with the game's design. As it currently stands, there really is no reason to build any defense facilities, besides maybe a mind shield, because it's more effective to defend your base with a small squad equipped for the job. In the 20 years I have been playing, never once have I thought to myself, "Defense facilities would have been a good idea".
It's not about taking ideas from other games, it's about tweaking things to reduce redundancy in X-Com, which has always been it's largest fault, by a long shot, in my mind. I'd love for defense facilities to be worth considering. As it stands, they aren't even worth taking up space in my base for, which is a shame.