Here is a draft I've had sitting un-posted for a couple of weeks. I'll post it here with a couple of edits to let people know what I'm thinking with this list.
Wall of text warning!
Executive Summary: the Crassus Infantry Formations need a boost.
Cut to the summary at the bottom if you can't be bothered!
The following is a theoretical firefight between a Steel Legion formation in Chimeras and a Heavy Mechanised formation in Crassus, where both sides are all able to shoot, and are not in any cover, with the Chimeras/Crassus tanking the first hits.
Steel Legion:
12 infantry + commander inflict 4.329 hits
7 chimeras inflict 2.331 hits
After saves (4 on Crassus) = 3.66 unsaved hits (1 on Crassus).
Heavy Mech Remaining = 14.33 units.
Heavy Mechanised:
12 infantry + commander inflict 4.329 hits
2 Crassus inflict 2.0 hits
After saves (all on Chimeras) = 4.22 unsaved hits.
Steel Legion Remaining=15.8 units
So heavy mech inflict 0.5 more unsaved hits but are outnumbered at the end by 1.5 (or there abouts).
The Chimera formation have the other benefits too like larger formation size, less prone to losing half their formation when one tank blows up, have an AT ability and are faster.
The big negative in my mind (as has been pointed out before) is all eggs in one DC2 basket (well, half the eggs) rather than the firefight ability. Lose a Crassus full of troops and the formation is toast.
I'm thinking of removing the crit from the Crassus completely (which will help 17% of the time on the first unsaved hit inflicted, so not much!).
I don't really want to mess with the Crassus stats, as they are what they are. It isn't really big enough to warrant DC3.
Note that some upgrades REALLY help them out too. Tanking a few hits onto some LRBT's significantly reduces the formations vulnerability (at the price of -5cm movement, plus 125 points) and means that people need to dedicate a fair bit of shooting to take out the Russes before having a go with the TK shots they have.
Cheaper upgrade costs for better synergy might help (I have already shaved 25 points off the Russ upgrade for this reason), though that doesn't improve the core formation, so I'll try the cheaper Crassus too.
The Fire Support Platoon was a bit of an unintentional Easter egg hidden in the list. You get another Crassus with three empty transport slots. There is also another empty slot in one of the other Crassus. Put the largely empty one at the front of the formation with three normal infantry in it, and distribute the heavy support units in the other two Crassus with the other infantry. If you lose the one on point you only lose three infantry that were in it (so only 6 BM). A gamey and unsportsmanlike tactic, and surely not one distinguished Epic players would attempt!
As for AA, I'm still thinking about different options. The flak upgrade is largely in there for people who don't have praetor models or proxies yet, and is probably on borrowed time.
For the Sky-Killer platoon:
A formation of two, with the Pilum warheads being:
R60cm 2xAT5+/AA4+ 200 points.
2x AA hits per turn (so AA is the same as 3x hydra), more resilient than 3x hydra, so fits the list meta of bigger, stronger, fewer.
And/Or
Run the Sky-Killer as a 0-1 upgrade for all of the DC2 formations in the list (Praetor, Dominus, Macharius, Omega and Core Infantry). Easier for me to balance but a no brainier upgrade that would cover the whole army under a reliable AA umbrella.
I do like the idea of a stand alone formation for this army as I think it suits the theme more (formations of big specialists).
Some other random points:
Dominus have 2BP each, (not 6!) and don't have slow firing. As it is a triple bombard, in theory it COULD be 3bp each without slow firing. Like Jimmy said earlier, it's hard not to go OTT with this unit. What do people think about that?
I thought Ogryns combined with the 4+RA of the Crassus would make the infantry companies too resilient in engagements. Perhaps that's a good thing (given the comparison above), and suits this list. 3x ogryns plus one Crassus would be about 125 points.
Formation and upgrade points for things like Leman Russ and Macharius are based on other approved lists.
I'm reluctant to change them, but I did reduce the cost of the Russ upgrade by 25 points (when compared to the Steel Legion upgrade, which is 50 points cheaper than the Krieg one, probably due to relative transport speeds) to try and encourage it to be taken.
I note in the new Krieg list that the Macharius formation can be upgraded to 4 which makes them pretty interesting and is something I shall steal.
Centaurs are a great way to get fast Stormtroopers so I haven't really considered Valkyries.
I deliberately left out single warhounds and took the plasma off the paired formation. Because I wanted activation count to be fairly hard to manage in this list, and macro can be spammed too easily with plasma warhounds and the omegas. I may have over reacted in doing this.
Summary.
So the next draft's changes are looking something like this:
Reduce the Crassus points from 75 to 50 each (13x infantry and 2x Crassus = 350 points) with either no Chimera or +50 point replacement Chimeras option.
Possibly Make an Infantry Platoon upgrade:
6x infantry and 1 Crassus for 150 points.
Change the Fire Support Platoon upgrade to:
6x Heavy Support and 1 Crassus for 200.
Possibly add an Ogryn Platoon upgrade:
3x Ogryns and 1 Crassus for 125.
Change the Sky-Killer Platoon: Formation of two. 200 points.
Pilum warheads: R60cm 2xAT5+/AA4+
Change the Dominus' Heavy Bombard to 3BP (so the unsuppressed formation of three has 9bp).
Change the Heavy Tank Platoon to a Company.
Make the SC an 0-1 upgrade.
Remove all footsloggers.
Possibly add in a Chimera Infantry Platoon.
Thoughts?
Also thanks for the feedback everyone! I feel like this list is developing nicely thanks to some quality input.