Nah. As far as I know, there's nothing inherintly "concrete" in any given moden FPS engine that won't allow transforming mecha. If programmers can do it on a console, why not a PC game? Besides, most of the big games are released with full studio developer kits. The problem as I see it is tht any such mod would likely never be completed, as most mods seem to just die after a year or so of work. But if a talented, commited team worked on this I don't see why it couldn't be a reality.