Now, before you burst into utterly hysterical laughter at the concept, think about it. This movie could give the whole series concept a greatly needed rework! Okay, stay with me.
Remember the fact that Captain Planet is supposed to be an emissary of Mother Earth, Gaia? Well, what if instead, Gaia did not actually show up in person, and instead, each of the five kids (Wheeler, Ma-Ti, Kwame, Linka and Gi) found shrines or temples in their respective home lands dedicated to an elemental nature goddess from that land's culture?
Within those temples, each kid finds a ring, corresponding to one of the elements. For example, Wheeler is Fire, so he'd find the temple of a fire deity in North America somewhere; Kwame, an African earth deity; Linka, a European air/wind deity; Gi, an Asian water deity; Ma-Ti, a South American life/fertility/wilderness deity. Each member would only see/interact with their 'patron' spirit.
And, at first, his issues would be against deforestation, or pollution of the skies and seas, or overfull landfills, people not recycling and being green, and natural disaster relief. But his foremost foes would be an advanced and deadly alien race, on a mission of conquest, who see the planet as ripe with resources (wind, water, solar, etc) that its inhabitants never make proper use of anyway. Not to mention, a whole planet full of human labor. Score, for them, if they win. This is why the villains that we all know from the show (with new, slightly less on-the-nose names), are working with the aliens, in hopes of being spared enslavement, and getting in on the profits.
And if he absolutely must have a ridiculous weakness, rather than have him grow weak and weary in the presence of pollution (I mean, really, he's strong enough to stop polluters and other defilers of the planet, otherwise he's really helpless to save the planet he's here to defend), he might have no effect on processed and artificial elements (ie plastics), or be weakened by prolonged absence from his planet. Being teleported into the depths of space, for example, would render him gradually weaker, and even this would take a while to weaken him.
Perhaps his powers could even be somehow tied to his personal connection with (or the willpower of) the "Planeteers". For example, there's a theory floating out there, suggesting that Ma-Ti has the most necessary of the powers (and that, in his absence of the element of 'Heart', Captain Planet would lose all empathy and compassion, indiscriminately attacking any and all threats to the Earth's ecosystem with all manner of natural disasters. Frightening concept, if you ask me).
Am I being delusional over a series that I really enjoyed in my youth, or does the superpowered eco-friendly do-gooder really have the potential to give 'going green' a good name?