So this is just something that id always thought would be cool. Basically, every instance would have a special pack of monsters that, when killed, the player collects. The player can then use the monsters to place inside a player made base. Players can increase their base size by defeating other player bases. Bigger bases means more loot, and more monsters. Every base has a boss room, and the bosses are captured map bosses (but not things like guardian level bosses or above). Whenever a player dies in their base, or kills the boss, the base owner gets some sort off currency that allows them to combine or evolve their monsters/bosses. By endgame, the players can evolve big bosses, maybe at the same difficulty as a guardian. These bosses would have unique drops. To incentivise building a big base, you can only attack bases at a similar level to yours.

Idk its something ive been thinking of for a while, and its kinda inspired by the original Mighty Quest for Epic Loot game, where you did something kinda similar. Obviously this is not as comprehensive as i could put, but im limited on time.