There are two main ideas I would like to propose: Co-operative battles, and continuous battles.
Co-op battles is the idea of having multiple people working towards a single battle, which I think has two possible variants.
The simpler option would be to have an enemy team with a very large amount of health and or armor which doesn’t re-set after each fight, and that is consistent across all members of a group (likely a guild). Each participant would be limited to one or two attempts per day, with rewards given to all participants. It might be advisable to have each enemy gain a moderate amount of armor at the start of each fight, to mitigate the power of sheer numbers. If the enemy ends a fight with higher health or armor than it started the fight with, it should re-set to the lower value, to prevent weaker players from worrying about their first attempt.
The more complex option is having each players team act as a single unit in a team of four players. A unit on the players team would be selected to act as the “face” and fill one of the four slots in the co-op team. From here there are two options: either have that be the end of it, or, when a unit is killed, replace it with the next unit from that players team. Each battle of this type would have one player act as the leader, who would control the entire team, but rewards would be given to all participants, with a preference to the leader.
Continous Battles is the idea of facing multiple teams in a row, without resetting your team. I feel like this one is quite self explanatory in how it could be implemented.
Feel free to comment on which options seem interesting to you, or suggestions for alternative ways to implement them.