"Just a random idea. There is a backstory if you care...it's a little scatter brained but it's still good. 8')

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~~Backstory~~

The Medic recruited all those other guys in the picture and the person they're sticking the needle in is their new recruit.

The Medic is a scientist who has altered human chemistry to make super-humans. His goal is to over take the human race. Their powers are based on some of their strongest abilities that they had as humans.

The Medic is essentially a telepath and a super genius.

The Sniper has incredible sight and can see into the future based on the choices someone's made.

The Demoman can shoot these sort of magical projectiles that are like bombs and he can time their explosions.

The Engineer can bend the will of nature to build things or as a weapon.

The Spy who is the only one with more than one or two abilities. He can change his appearance, he has the ability of invisibility and can bend the will of someone else however that last power is fairly weak but still viable.

The Heavy has super strength and is bullet proof.

The Soldier he can fly, and shoot projectiles from his hand that explode on impact.

All of them have a super regenerative ability that keeps them young/healthy, and allows their body to repair fast from injuries of all sorts.
"