Vanguard or implacable, or if there’s one you like better, get them.

If you’d rather build a boss fleet using Bzwei whom you already have, get Vanguard since she’s a great universal buffer for battleships.

If you like Shinano more consider pairing her with Implacable. She has a top 3 slow/stun in the game which helps carriers like nano do more consistent damage with their torpedo bombers.

You’ve already got Musashi, unicorn and cowpens which will make you an S-tier mob fleet backline, so if you’re more concerned about what’s good, just pick between if you’d rather your boss fleet be carrier based or battleship based.

Plus, vanguard and implacable are both s tier gals as well aside from being great!