The problem with upgrading the CV is that the Essex was the ultimate wartime fleet carrier, your next progressive step in carrier design was the Midway class, but none of them saw combat in WW2 (Midway and FDR were commissioned in Sept. and Oct. of 1945).
The problem that I see - is that any TG with a BB in it will make mincemeat of a normal CA battlegroup, unless each country's BB group specifically seeks the other out. Give the size of the maps, that's probably unlikely. The commonality of the existing CVs groups mean that one group typically doesn't have an overwhelming advantage over another, the notable exception being those maps that offer up dual-CA groups.
Perhaps - and I'm just tossing this idea out - the idea of having a single ship, perhaps a CB or BB, as a player-spawned surface raider, might have some merit. Eliminate the auto-puffy, have the AA controlled in the same manner as a bomber (all guns fire that can be brought to bear), and have it cost perks. It spawns from a port, and must be landed with range of a port.
While its true that most players won't spend huge perks to take a BB or CB out 'carrier hunting', it could certainly make for some interesting gameplay. The downside I see is no one upping to protect a single-player controlled ship like they do a CV.
Just a thought. I'd certainly enjoy tangling with a CV group.