I think the sub plot should be optional, but present. There are already rules that state it can't be squadroned with other cruisers, so that shows it's "one step outside" the rest of the fleet.
It really does "fit the bill" without any major changes, it's only one ship per fleet, and it doesn't really effect the feel of the fleet. It also happens to have all the benefits of a battlecruiser, battle barge, and strike cruiser with none of the negatives (45cm batteries, torpedos, 6 armor, t-hawks, 25cm speed, fast moving, great leadership, dorsal bombardment cannons)
One specific thing that needs to be done is a rules clarification that it gains the benefit for the space marine boarding 2+, though odds are the boarding will be done by Sisters of Battle
Space marines don't get lances, aside from the nova. Sorry. Bombardment cannons are JUST Fine.
I agree with the others that Doctrines at this scale really don't effect battle in fleet formations. Those are represented by the stellar LD that space marines get. Again, Space marines are NOT special at this scale, this is NOT 40k, and this is NOT their natural realm of combat. There is ONLY ONE space marine chapter focused on fighting in space, the Astral Knights, and they were nearly eradicated in a fight against the necrons.