[SUGGESTION] Weapon: Ice Burster

This may be interesting... But only annoying if badly implemented...
I also agree that if such a weapon is implemented, it should be a mutator first, this would allow people to test, and say what they think about it. Fixes and suggestions could then be made by everyone, leading to (maybe) a very nice feature of Xonotic.
I also agree that this would slow down the speed of the game if not implemented correctly. Complete freeze should, I think, be used with extreme caution. For the "go slower" part (2/3 of normal speed for example) it could lead to more tactical playing. I also think that this slow down could affect the speed at which you reshoot. Giving you a small, but possibly fair (you took the time to use the freezing weapon...) advantage.

Few more ideas:
1. Not an high range weapon (you should be able to know from where the shot come.)
2. Take time to reload, need to load when you take it before it can be used. (So that the freeze gun would really be about tactics and not about spamming everyone in your way)
3. If it completely freeze, the freezing time should be really short, lets say 1-2 seconds maximum as said by others (maybe just a stopping power, drops your speed to 0 if you are running, or something like this)
4. Very minimal damage, if any. ( Since you would probably be slower after, that weapon should not be the one that always bring everybody to death. Example: Slower+freezing damage going for a few second+the crazy guy in front of you shooting with his very big weapon)
5. Weapons that produce heat reverse the effect of freezing. (Not wise to shoot with grenade launcher, rocket launcher, etc... because your enemy would get back to normal speed and control because of you. Again potentially more team-play here)

Hope this helps