My thoughts are that any AR wouldn't be different enough to be worth the effort to choose between it and the SA80.
My thoughts are that any lever-action/bolt-action/semi-auto rifle wouldn't be different enough to be worth the effort to choose between it and the winchester.
My thoughts are that any rocket-launcher wouldn't be different enough to be worth the effort to choose between it and the LAW.
My thoughts are that any dual-barreled wouldn't be different enough to be worth the effort to choose between it and the current one.
These are weapons that serve a purpose. Another AR would preform basically the same as the SA80, especially if it's still 5.56. Even if you put in a 7.62 rifle, it's still basically the same rate of fire and all, just less accurate, more recoil, and more damage. It's still basically fufilling that well-rounded role.
These are things that only add to the visuals of the game, and not the gameplay. Those are my thoughts.
A machinegun would maybe be nice but it doesn't fit with KF because you'd move so slow and be taking so many kills from your allies when you didn't have to worry about movement.
A grenade launcher is really the only thing that is lacking. It's significantly different from all the weapons.
And incendiary grenades could be nice as long as they don't make the flame thrower useless. Flashbangs or some kind of gas would add to the gameplay too.
More specimen types could add to the gameplay(dual-sword-arm gorefasts, bloats without axes that belly slam you, maybe stuff like that) a lot more time spent making new weapons that modders can easily do.
While modders can enhance gameplay with randomized maps, objectives starting on waves, story mode type stuff like "The Hive", they are more 'important' to me, because they enhance the standard gameplay more than added weapons do, and it's VERY easy for modders to just add in new weapons but not as easy to incorporate a randomized level and objectives system.