Can this even be done? Are vision ranges exposed to scripts? I don't think so, but I can't be sure...
It'd probably be a very useful engine change if it isn't already possible.
In general, actually, a full depth pass on night fighting. Night battles absolutely suck balls, so the more equipment we have to even the odds, the better, in my opinion. In the early game, I'd like a variety of flashlight options. A head mounted flashlight would be the most basic form of night fighting aid. Throwing magna-flares is a waste of precious TUs and I loathe doing it.
I'm not sure if this would be possible without engine tweaks but I'd really like some sort of illumination parachute flare that could be shot into the air and provide wide area illumination. Perhaps that could be fired out of a 40mm grenade launcher. Ideally, just one should cover a really wide area. Though I can't think of any existing weapons that work like this so I really don't know if it's even remotely possible. Basically I want a very powerful floating light source. Perhaps a floodlight mounted on to a drone?
I also think it's very weird that most interiors have no light sources whatsoever. It'd require a pass to add lanterns to all the different map tiles but this would add a lot of depth to night fighting (you need to stay away from light sources to remain concealed)
If NVGs become a hat slot item (and I think they should become one) there should of course be a wide variety of them. They could be found on enemies as well. And I don't know if it's possible to tweak the vision cone of a unit but ideally different NVGs could limit a unit's vision cone. High end NVGs can grant thermal vision or bonuses to targeting (I'm thinking like the ENVGs, used by the real life US army, which actually highlights detected thermal targets). Then - and this is really getting into wild territory and I don't know how feasible this'd be to implement - early NVGs might rely on IR illumination lamps and increase a unit's visibility against anything with Thermal Vision.
I don't think I've ever seen a turn based strategy game try to even broach the topic of night combat. I think sneaking around without being seen is a fun mechanic.