I have no problem with the Doomwheel ^_^ though the remake of it.. ugh.. awful.. Been too long since I played and used it in battle.. In 4Ed, the warp lighting is pretty random based on dice rolling. As long as the Engineer is still alive, you can choose if you want to release the lighting or not (if he is dead, it's completely out of control and will randomly shoot). Basically, an even number is a sucessfull attack and that's it's strength. If it's an odd number, it's a misfire and can pretty nasty (1/3 chance of attacking itself, 1/3 chance of just damaging the generator so it loses one of the three shots, or 1/3 chance it jams the movement so it moves in a random direction). Like I said, if the engineer is dead, it rolls uncontrolled and shoots randomly.
for a custom one... mm.. I think someone on a blog someway commented about it reminding them of the Cleaner machine from Labyrinth. I'm not sure myself but.. I can see a nice idea from that..