I like this, but you would, naturally, need to expound upon the idea quite a bit. Similar concepts get tossed around pretty frequently here.
Obviously, you'd want to include changes that allow haywire, armor bane, and melta to work against against vehicles with T instead of
AV. I suggest something like:
* Haywire = poison for vehicles; no devastating blows
* Melta = fleshbane for vehicles while within half range.
* Armorbane = fleshbane for vehicles
I also like the idea of introducing a "Mechanical" rule to the game that can be applied to all vehicles, necrons, crisis suits, etc that makes them susceptible to the above rules but immune to the effects of fleshbane, poison, etc.
I'm also not sure we really need three different values. Provided the vehicle's toughness is under 8, strength 4 models will still be able to hurt them in melee on 6s just like now. Personally, I don't find that armor facing adds much to the game (aside from making vehicles melee-able), but I do find that disputes about armor facing can be annoying.
I like the once/turn pivot idea in general. It would be a cool addition to normal
40k. I just think I like the idea of having a single toughness value a bit more. If the multiple toughness values or armor facing systems were kept, I'd enjoy playing with the 90* pivot rule. That said, why only give it to walkers? It seems like a skimmer would probably be agile enough to adjust its facing in response to enemy fire.