A Space Phantom has been watching the Avengers, gathering information on them behind the scenes. It has often set it's sights on the heroes who save the day from certain destruction.
For a while, it wanted to take the place of one of their own or even play a game with them until it finds a suitable form. It would at least be something worth doing anyway.
The Space Phantom has the power to become anyone or anything, such as an object, person, animal, or super-powered person, and have access to their powers and roles. When they do take a form, the original form or person gets sent back to their dimension, trapped there until the imposter takes another form. They could switch to a new form without anyone knowing unless they fight back.
A Space Phantom comes from a dimension with many Space Phantoms, where they would pop up in the world and carry out their own plans. Because of this, they can be tricky and cause trouble.
As soon as it gets the perfect form, the Space Phantom plans to experience their lives and the joys they develop. Since it can be anyone or anything, a lot of opportunities are there for it.
At first it thought of impersonating someone, but instead, why not become them while they themselves can't help but watch. It could become their body, controlling their actions while their souls are helpless but be passengers in their own form. Maybe that will get them riled up and willing to go crazy by fighting others. Or perhaps, it can be their clothes or items and experience being quite close to them and act in strange ways.
Either way, the Space Phantom plans to use it's powers for their own needs. This will be a wonderful way to mess with the Avengers.