I'll give it a go
General Iroh is one of the most powerful firebenders, he's definitely too powerful.
Not completely sure about the Master Chief(i'm doing some research on that), but the rest of them are good.
Legion of Doom:
Leader: Dr. Doom
Dr. Doom is definitely too powerful, and Tempo falls under the same category as Quicksilver would.
The rest of them are good, but Madison Jefferies will not have the Box robot.
Keep in mind that everyone is allowed to change their characters at any point, up until the tournament starts.