Could definitely be an interesting mutator. I was just thinking about a freeze tag game on skybox maps. Here are 2 examples I thought of:
Regular Freeze Tag.
1 player for every 4 gets the freeze gun, the rest get lasers. The player with the freeze gun is obviously trying to freeze everyone, and the other players are trying to get away. The normal players get a laser. This can be used for 2 purposes. 1. Thaw frozen players (requires like 3 hits or something to unfreeze) 2. used as knockback and/or stun against freezing players. Could possibly even the odds a little bit by giving freezing players some slight transparency to make them harder to see since there is a discrepancy in team sizes. Points are earned for not getting frozen, knocking freezers off maps, unfreezing players, or for the freezers, points are earned for freezing players, and a nice bonus for freezing all players within a round.
Fire & Ice.
Teams are even, All players have a laser and a freeze gun.
If you get hit by freeze gun, you take a little bit of damage continuously, Your movement speed is slowed.
If you get hit by the laser, you take an initial burst of damage, and a medium amount of damage from then on. Your movement speed is NOT slowed.
Both effects can wear off over time. Also, being hit by a projectile from the opposite weapon (frozen person gets hit by laser or burnt person gets hit by freeze gun), the effect ceases.
Players earn points for the points of damage they cause to players on the other team, and for removing effects from teammates. Players lose points for damage from team shots. They can undo those effects before they lose too many points though.
The only problem I'm looking at is with the second one, and that's players shooting the ground beneath them, which would undo the effects placed upon them by others (like when you use the laser as a portable jump pad, and you take damage. This wouldn't be an issue on the first because the players are completely frozen, and not just slowed down. This should be able to be remedied somehow for the second one though...
Just ideas for mutators or as a new game type. I do not believe however that the freeze gun would be a healthy addition to normal game types. On its own, or with the laser though, I think would be interesting.