Sadly I have no ideas either.
No worries!! I did a small search about Amazon Jungle in Marvel Comics!!
In the game that stage was used for evil She Hulk!! Since I already have a stage for her we can also use it for:
*Masters of Evil (Baron Zemo, Radioactive Man or even Wrecking Crew (Thunderball and Wrecker)
I just mentioned chars we have in Mugen now!!
Story:
Captain America parachuted into the jungle looking for Baron Zemo, defeated his guards and shattered his primitive palace with the vibra-gun. While Zemo attempted to escape by plane, he magnetically attached his shield to follow him back to the United States of America.
Zemo returned to his hideout with the Enchantress and the Executioner to plot their revenge against the Avengers. They recruited inventor Simon Williams and bombarded him with the most powerful ionic rays to make him the Wonder Man. After having earned the Avengers' trust, Wonder Man got back to the jungle with the Wasp, who was taken captive: the Avengers came here to save her, only to be betrayed by Wonder Man. The Masters of Evil came very close to defeating the Avengers, when Wonder Man decided to betray them, as the Avengers did all they could to save him, even though they didn't know him. Wonder Man saved the Avengers, then subsequently died from the poison inflicted by Baron Zemo, who managed to escape through a secret passage with his allies.
P.S. Hope u release a Mugen 1.0 version too like the Boston Aquarium stage!!
Veterans Day