I've some ideas...
One can be the freezer - freezes a player or monster for some seconds, it don't takes any damage, it only can't move, jump or shoot.
Concussion Rocket - Just like the TF's conc grenade effect.
Illusionary Monster - Spawns a non-solid monster, the monster attacks you, but it don't do any damage and your hitbox isn't blocked.
Hallucinogenic Grenade - Just like the TF's gas grenade, but with no smoke that does damage.
Summon - Spawns a friendly monster, the monster can kill, but you do nothing.
Translocator - Throws a teleporter pad, press jump to teleport. It can telefrag, but no problem.
Sucker Slipgate - Is an orb that pushs you to its direction and when you touch it, you'll be teleported to a random info_teleport_destination point.