Actually, there is only one "rare weapon" in game, the gauss pistol. I've noticed that this weapon, ultimate for snipers, very rare so very precious, was adding really a great new things in PvP fight. Like it is more powerfull that normal stuff, even the better you can get trough usual ways, but not protect the player from bullets and does not insta kill any opponents, it is a very interesting weapon to have, hard to gain but easy to loose. It add a big interest in fight: usually, you loot players, and that's cool, you get more stuff for next battle, and if you die, it's annoying, but if you have some stuff in your base, it's not so bad. With gauss pistol it's different: if you loose it, you really actually loose something, that will not be easy at all to have again. But when you loot one... So much pleasure, it really have a sense, it will make a real diference if you use it in next fight. Also it add a challenge, because ammo are rare, even if you know where to find them (I won't spoil ;p), and you can count each shot you have and you have a risk to not being able to use your awesome weapon because of lack of ammo. Rare weapon add a real taste to victory, a good pleasure to play, tough it add more rage in defeat.
But what I find sad is that it's the only rare weapon. It's usefull for sniping, but a bit waste to use at close range. Why only a small gun?
So I suggest to add other special encounters, with other rare weapons. Vindicator for exemple, would be a very powerfull big gun, but not so ultimate cause of lack of range and... rare ammo. Like it use 4.7 caseless, another ammo not IG, you could add a way to obtain this ammo (maybe the same as 2mm).
There is also the bozar, a sniper in FOnline. Like it is less powerfull than other rare weapons, but more than usual ones, I think we could leave it with .223 ammo, and make it appear more often. It would be between the usual weaponry and the rare weaponry.
We could also add the HKG11, and small gun burster will be happy with this deadly bursting weapon!
The last problem is for energy weapons. A rare turbo plasma would be awesome, but sadly, it use the same MFC than usual weapons! 2 solutions: leave it as it is, and it won't be so bad, MFC is expansive anyway. Or we could create a new ammo for this weapon, let's call it "overcharged MFC" or whatever else and make it available as 2mm and 4.7.
It really think it would add more taste to the big fights, if they still remain as rare as gauss. Add a real sense to victory and defeat...