What if each month some subset of the characters were the only characters available in a particular Arena mode?
Not replacing GA, not replacing Arena, I understand some people have spent a lot for that or have their favorites.
But there could be an arena mode with just a subset of characters that would force players to think again, come up with some fun squads on their own, and try them out.
This (creating cool squads and combos) is what brought many people to the game. It’s like creating your own deck in Magic. Getting fed a squad (like Phoenix) is not nearly as fun.