I thought of something similar which would be a bit like a Zombies or Crucible type mode (modes of other games)
Everyone would get maxed heroes to use - like the ones in Hero Spotlight.
Each week you get a random choice of 10, from which you can select 2 at a time.
There’s only 1 battle, with endless waves - this reduces the faff of clicking around that we have in Breaker quests. The battle can be paused at any time and resumed on the same wave.
The waves are composed entirely of creeps (not heroes) of increasing number & strength.
The goal is to reach as high a wave as you can over the course of the week without your heroes dying. Once you’re all dead, the game is up. At wave intervals you can receive upgrades that you can apply to your heroes such as bonus healing, extra damage etc or (very rare) revives - these can either last for a limited number of waves or be permanent for the week.
You also receive permanent rewards every X waves to be used in general play.
At the end of the week, you get rewards based on how far you’ve got. The mode should be designed in such a way that no strategy exists that could beat an infinite number of waves, and general play rewards should be scaled to the player’s Team Level.
Manual skill activation is possible (ie. not like Arena/Coli).
This would be fun and also of potential financial benefit, as people would get to use heroes they’ve not tried before and see their potential - and might then spend resources to use them in general play too. There could also be a subscription-type deal like the Daily Diamonds for this mode, that delivers a perma-boost to the upgrades received during the mode and can be purchased weekly. It should not be possible to purchase deals that boost the general play rewards directly - only the mode consumables, which indirectly enable more general play rewards to be received.