I'll just add that the team should be careful to make the mod different from all the "aliens vs marines" games out there ESPECIALLY Natural Selection.
Ways to make Exterminatus more "40k-like":
*Stick to the idea of having hands and weapons grown together.
*Make all 'Nids variants of the same basic design. It's like that in the new tyranid edition, everything have 6 limbs and bone plates along the head and spine.
*Implement Synapse control and Natural Behavior.
*Having a "horde" feel, both big and small creatures.
*Probably less stealth than usual for aliens.
Exactly what I was thinking. Nids vs Marine would be to much like Star Ship Stroopers in my mind. To me Orcs just make is unquie. Not many games do you see orcs welding rocket launchers or a gun of anytype that is. But you said you had it planned out hope its good.