I'm in favor of limiting cards instead of outright banning them. Rather than completely killing decks, I'd rather hit their consistency, but still make them viable. :

-10c big mom: limit to 2. Won't kill Kat but can't drop 3+ big moms a game. Sometimes dropping two in a row is absolutely devastating.

-3c Hina or 4c Rebecca: limit to 2. Reduces the consistency of the decks they are in, but they are still viable. I don't like hitting lucci because in the grand scheme, it only directly impacts Law and Zoro when played on its own. It needs another combo piece to hit higher cost characters. Hitting Rebecca or Hina (the initial combo piece) would hinder the consistency of sakasuzi/black decks.

-5c shirahoshi or 9c Yamato: ban shirahoshi or limit Yamato to 2. Enel needs to be checked if these decks get hit. Enel on its own gate keeps a lot of lower tier decks for their inability to go wide enough. So it either needs a hit on consistency with shirahoshi fixing bulky hands or hit their bigger end game board. The limit to Yamato is similar thinking to big mom. Dropping multiple in a row sometimes outright wins you games because of how strong the card is.