Mimic Variants For 5e
Variant: Multi-Mimic
Multi-mimics are mimics that can transform into several objects at once. Usually, only ancient mimics can do this, but there are some mimics that are born with this talent. Multi-mimics have the improved Shapechanger trait below instead of the normal one.
Shapechanger. (Multi-Mimic) The mimic can use its action to turn into 1d10 + 1 objects or back into its true form. Any equipment the mimic has does not transform with it, and does not count against the number of items it turns into. The mimic's stats remain the same in each form. A useful trick for a mimic could be to turn into a chest and several items in the chest.
Variant: Mimicolossal
Mimicolossals are mimics that are capable of transforming into huge objects and any objects that would normally be in them. Mimicolossals have the following improved Shapechanger trait instead of the normal one.
Shapechanger. The mimic can use its action to turn into an object or building of any size or back into its true form. If the mimic turns into a building, it also turns into any object that would normally be in that building. A mimic can only turn into a building that it has been in before. Any equipment the mimic is carrying is put in the building at a location of the mimic's choice. The mimic's stats may change at the DM's discretion.
Examples: If the mimic turned into a castle, it would turn into the castle and everything in it, including the mounted defenses. The mimic would have to have been in the castle before. If the mimic was carrying a sword, it could put the sword anywhere in the castle.
Variant Trait: Internal Storage
Internal Storage. The mimic has an internal space within itself. The space is infinitley big and can hold an unlimited number of things in it. When a mimic eats a living creature, it may choose to send it to the space instead. A mimic can structure the space in any way it wants (it could make its internal space a dungeon of rooms full of traps that it collected, or just make it one room.)