Stalkers are useless alone. They are designed to act as part of the ZED horde and will be more efficient in confined spaces.
A Phaser, (not a Phaser) is an interesting concept, but any implementation should have this ZED phasing through doors and walls, only if the other ZEDs cannot pass through. If players are not behind doors, it should act as a Stalker that is invisible.
This will promote Zerking though, and also cause problems for Zerkers at the same time. Probably not a good idea for KF, but interesting nonetheless.