To throw a left field idea into the mix; how about maybe adding an option for 0-1 Fellblade SM superheavy tank to the list?
The Imperium lost the ability to make Fellblades long ago and the background in the Imperial Armour book notes "only a few" chapters still have these ancient relic war engines by the 41st millennium. However back in the Heresy the Imperial Fists were known as one of the top few users of them out of the legions, with them "making extensive use of the Fellblade", so it's probable they'd have one or more kicking around for when needed. If included it should definitely be rare and a 0-1 rather than free choice. It would fit well with the theme of the list as a slow, powerful, tough defensive unit. The main gun is powerful, but it's only one shot on a tank that would probably cost 300 or so points. Some of us might have fellblade models and though they're in the heresy list it would be nice to have a regular epic list to use it in.
Stats would be something like:
FellBlade Superheavy Tank
WE 20cm 4+ 5+ 4+
Twin Fellblade Cannon 75cm MW2+, TK(1)
Quad Lascannon Sponsons 45cm 4 x AT4+
Twin-Linked Heavy Bolter 30cm, Fwd
Demolisher Cannon 30cm, Ignores Cover, FxF
DC4, Reinforced Armour. Critical Hit Effect: Destroyed. Units within 5cm of the model suffer a hit on a 6+.
