I don't mean to pick so nitpicky, but I personally felt that both the series and the movie ended on a low note. There were so many things that could have come out of the series, but it did not come to full fruition. Here are some of the major subjects that could've been in the series.
GOVERNMENT INTERVENTION
With all of the things that were going on in Beach City, where was the military? When a giant hand falls out of the sky, water disappearing from the earth, a and a pink Godzilla appear on the shore, I expect military hummers, police cruisers, government vehicles news vans, and choppers to the flood the city and approach the threat. This could have shown the relations to both Gem and Earth Authority and how it was starting to heat up.
HUMAN VS. GEMS
With all the crap going on on earth, There should have been a human disgruntled busybody who starts an anthropocentric organization/paramilitary who hates the gems and wishes them to be expelled from the Earth, establishing it as a human sovereign planet. They would commit major delinquency acts, recruiting campaigns, and protests before gaining terrorist status. They would see steven as an abomination and the gems as threats to humanity, wishing to destroy both the temple and little homeschool. With the tension heating up, they could gain political status and declare war on gems and those who stand by him
WISDOM FROM THE OUTSIDE
When Steven was having the internal struggles with himself, there should have been a human mentor that would teach him of the ways of dealing with the struggle of being human and how he needs to learn how to become self-sufficient and resilient with himself and those around him. He could have been the person who would cure Steven in his corrupted form.
Steven once said that he had the power of change to overcome adversity and learn from it. These topics would have been major building blocks to Steven's journey through his crazy life.