A few ideas

[JohnBeckman]

The first is the implementation of zip-lines, which we can hang from and whatnot.

The second is a weather system, with one or two maps which are heavily laden with snow, as that means we can ski, possibly use snowmobiles etc.

The third idea is stuff like thermal imaging devices, as we can get people to cut out the lights and have a gunfight in the dark or something like that.

The fourth idea is a sort of lockpicking minigame in which players can break through locked doors.

What do you guys think?

[Peoples]

Hate to say this but most of those are ideas I've already seen (I actually suggested thermal imaging myself recently). Ziplines are not practical, although that would be kinda fun. Lock picking is not going to work, since this is a game about the fighting, not the objectives.

[JohnBeckman]

True, but zip-lines would be quite nifty if you want to fire on enemies from down below. If that couldn't work, then zip-lines would be ideal for accessing neighbouring rooftops.

Furthermore, skiing and the use of snowmobiles would be quite nice, becaise it would make for intense driving and whatnot.