OoC: @Ven That sounds like a good plot, but I'd wait with implementing it. We have too many overlooming threats, so one more is just one more on the list.
Overall, I don't like when universes merge. Rosh's mechs have teleported from another dimension to get here, and there is a similar thing going on with your 40K stuff. In my oppinion, a good way to do it is how Nailik has done it with Kefka. That way, it isn't the enemies that are spilling over, but the technology. Granted, I don't know where the mechs will have for backstory, but I doubt William has the resources to build a couple of million of them.