As a mimic you are no more than a blob without an object to possess. As a bonus action you are able to implant yourself in an objects very atoms, becoming said object, however when ever you leave an object said object will break beyond repair. 

Depending on the object that is possessed, the mimic will gain different stats or buffs.

Due to being a cave dwelling race, you are able to see just fine up to 30ft ahead of you in the dark.

You are able to expand your tendrils to possess multiple objects at once, however just as a foot cannot untie the lace of a shoe from inside, your ability with each object is greatly restricted. The more objects you possess, the worse your control on any given one is. This move can ONLY be done when you are naked of any fully possessed items.

The classes consist of: 

• Dense: smaller heavy armors, ores, hammers, (things that are thick but not too heavy)
• Heavy: larger heavy armors, heavy weapons, large ores, full backpacks, chests,(anything both dence AND heavy)
• Light: leather armors, daggers/smaller weapons, arrows, paper, (anything that a child could easily pick up)
• Complex: crossbows, golems, constructs, (anything with multiple working parts)
• Welded: a dagger and a stick, (possessing two overlapping items at once)

Now you know the examples, here's what each class does(with room for alterations or additions as dm sees fit)

• Light: increases speed, agility, and gives advantage to reaction roles, however you lose 1d8 of health as long as you are in light form
• Dense: increases health by 3d6, gain resistance to bludgeon, slash, and pierce damage, move 5ft slower per turn and role with disadvantage on reaction roles as long as you are Dense
• Heavy: increases health by 3d10, gain immunity to all non magic damage other than fall damage, you are weak to fall damage now, you love 20ft slower than base speed, you role disadvantage on all reaction roles -1d4 from score while heavy
• Complex: gain ability {overdrive}, fuel not required, gain buffs or augmentations object had until next long rest
• Welded: better control over both objects with the ability to use them/segment them in new ways