I've refined my original idea to include the Night Lords as well as the Iron Warriors, with the Night Lords basically providing fire support to the Iron Warriors. I'm thinking of trying to paint them as they might have appeared during the Ullanor campaign.

Here's a rough draft of the list:

HQ: Terminator lord, 5 Terminators (I'm hoping to be able to mix standard terminators with Assault Terminators)

2 squads of Ten Marines with Rhinos
2 squads of five bikes, with power fists and flamers
2 Land Speeder Tornados with Multi-meltas
2 Predator Destructors (hoping to use Havoc Launchers)
1 Predator Annhilator (might try and use some of the Chaos accressories to give it a scarier and more authentic Night Lords appearence).

In a sense what I'm aiming for is an army capable of mounting a classic pincer movement in which the Night Lords can tear open enemy lines and the Iron Warriors then wipe them out with a tank rush.