I think Chameleon should appear in The Amazing Spider-man 3 because their is a story line in the comics which goes very well with the movies:
Chameleon works with Harry Osborn to create life model decoys of Peters parents, which shocks Peter and the people around him. This makes him feel that his parents are still alive but are made to complicate and mess with his mind. Or he could impersonate Uncle Ben to humiliate Peter in the same way, or Gwen Stacy too if she meets her demise in the second flick.
Chameleon cannot be working as a villain in The Amazing Spider-Man 3 on his own, as we still have the Green Goblin.
Green Goblin is one of the main villains in any Spider-Man story, he should also appear in the end of The Amazing Spider-man 2 to kill Gwen Stacy and leave a cliffhanger for the third installment but that may not be the case. He should destroy Peters world and people close to him, which will lead Peter to go after him and Osborn will meet his demise. However in the comics he cheats death many times so I think after his death Chameleon should take his place and work with Harry to create life model decoys of Peters parents. Then Green Goblin should return revealing he pulled the strings with Chameleon and Harry Osborn and reveals how he cheated death.
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