Well, I tried… really wish I could create a mechanic that will boost both naval bases as well as cruisers and battleships. Seems like AA capabilities is the only accurate solution but I doubt I would ever want that considering how much it changes the odds of opening strategies.
YG, you should make a list of potential SZ affected by opening round strategy.
You can have a special first turn suspended rule, such as no AA gun for BB or CA in first round.
On second round, additional rule start.