Haven't Battlecruisers already been established as Capital sized ships in Sins? Also correct me if I'm wrong but don't capitals already focus on dealing with other capitals and on using abilities? The strike craft/frigates/cruisers are the workhorses of the fleet.

That aside, what you're describing just sounds like a beefed up Heavy Cruiser, with the same tech level even(4/5), I don't see the point.

I agree that adding yet another ship class would be rather pointless. We basically have a good counter for everything at the moment (except for titans maybe). Adding yet another ship class would make balancing Rebellion even more of a pain in the butt.