I would like to recommend long and realistic sight ranges, for X2. I have seen this work very well in Xenonauts 1, and think it should be part of the sequel.
So far as I've seen, short sight ranges don't accomplish much, aside from making it necessary to research the technology of glasses. In a night mission, it makes sense to have limited visibility, and so you can have a terrible feeling of claustrophobia. But in the day time, it doesn't make sense for aliens to stand outside of your sight range and shoot at you. It's one thing for aliens to hide behind cover, but it feels silly for them to hide in plain sight (they kind of stick out).
Sight Radius
A lot of people seem to agree they want a reasonably wide vision cone. How wide is difficult to say, but I figure the maximum would be close to 180 degrees. Between 120 and 180, I'd guess.
I forgot to account for tunnel vision, a serious issue in battle conditions. 90 or 100 degree vision might be reasonable, in that case. However, short range peripheral vision, as Frutz suggested, might be nice.