Alredy discussed something similar to this.
But, if we’re not getting new Solo Adventures any time soon (the possibility has apparently been discussed, but there’s nothing definite in developement) I would love it if they woukd do a sort of Mythic Mode; where they cherry pick the fan favorite bosses and let you fight against each one. The general design can be the same, while also providing maybe new phases to keep the fights fresh, but their decks, Health and/or Armor are upgraded significantly to make them a truly timeless challenge.
As for the rewards, it doesn’t have to be anything fancy. But with how they’ve said that they are focusing on giving us more customisation options, having some of the highest quality ones (skins, unique boards, the apparwnt new “pets” ) be unlockable only by beating these insane challenges would be a cool way to give casual & hardcore players something to work for other than climbing ladder
Hell, you could even make these things into raids for the day Guilds in Hearthsone become a thing if that’s in the cards. Where parties of players all work to defeat a massive, ultra-powerfull boss. Not unlike the Lich King Fireside/Tavern Brawl event from ages ago.