An idea I’ve been kicking around for a while is a Slay the Spire-type gamemode. Select some starting troops from a pool (similar to Arena) and fight your way up the “Spire,” gaining new troops and weapons as you go. You could even have a system similar to Slay the Spire’s artifacts, where you get certain buffs (such as potions used in other gamemodes, as well as unique boons) after beating bosses. After certain challenges, you could add a Legendary or Mythic troop to your team, with random options from a pool. Each week, the mode would reset, and you would start fresh with a newly drafted team.
A mode like this would really highlight the tactical team-building many of us love about the game, and I know I’d be playing the heck out of it!