Commoragh's a pretty huge place, lots of resources etc., and plenty of madmen in control of it all - big murder-machines are kind of a given!
Gameplay-wise, most of the Eldar super-heavies would probably work out for them - it's pretty easy to argue that the Eldar flyers/titans are pre-fall designs (plus, a Dark Eldar Phantom would look awesome).
That and Tyranid Super-heavies/Squiggoths to represent the greatest exploits of the Haemonculi and Beastmasters.