I'd like to propose a mechanic for boss raids that I think could be quite fun, and would use some existing mechanics that may make it easier to develop.
The idea is to implement clan boss raids that are available to each guild, each season. To take down a boss, guilds would have to keep attacking it until it reached zero health. Bosses would have enormous health and armor and would be very strong, meaning it would take many attacks from many players in order to take them down. When attacking a boss, any cards used by the player would go on cooldown in a way similar to Survival. The cooldown would be specific to the boss raid so as to not discourage players from participating.
A leaderboard would be implemented to track the contribution of each player in the guild. It would show total damage done, the highest damage in a single run (perhaps with a link to that battle for others to see how it was achieved) and the reward for that player's contributions.
Rewards
Rewards could initially be in the form of Glint and/or Merits and include other things like cosmetics or even CC of a special card in the future.
There would be individual performance-based rewards as well as a reward to the entire guild when the boss is taken down. Both individual and guild-level rewards would only be awarded when the boss is defeated.
Perhaps there could even be a competition to see which guild can take the boss down first, with additional rewards for that guild. This would encourage high participation early each season.
The economic impact of handing out additional Glint/Merits likely wouldn't be too significant, but would still make it sufficiently interesting for players to participate.
Cooldowns
Cooldowns would be similar to those in Survival mode, where individual cards defeated would be unavailable for a set period of time determined by their level and foil. This would encourage players to collect and combine more cards to contribute to the boss raids.
Stats
Boss stats and abilities, as well as rewards, would be based on the Guild's tier.
Determining the exact boss stats would likely be the hardest part of the design of this mode. Bosses would need to be hard enough that a single player couldn't easily take down a boss, but shouldn't be so hard that the majority of guilds find it impossible. I would veer on making them HARD (at least initially) to ensure it feels like an achievement when a guild finally takes down a boss. If guilds were able to finish a boss on day 1 of a season, it would take away from the experience.
Boss rotations
To ensure the mode feels fresh, I think the team would need to ensure they rotate the boss every so often, and possibly have multiple bosses available at some point that guilds need to choose between (or to give greater challenges to very powerful guilds).
While this would in fact be a brand new mode, it would re-use a lot of logic and even screens from existing modes, which is why I feel it shouldn't be a very big effort (compared to some of the other things the team is developing). It would give a fresh &* fun place for people to go use their cards, would encourage collaboration between guilds, and may even foster competition among guilds.