The 100 Season 5 was discussed at the 2017 San Diego Comic-Con. One of the topics included the next major theme. Executive producer Jason Rothenberg teased that it has to do with the "battle for Eden," the one part of Earth that is not a wasteland. Also, Eliza Taylor talked about her character, Clarke, and her journey following Praimfaya.
Possible spoilers might be ahead. Do not continue reading unless you want to know what to expect when the series returns to The CW in 2018.
According to Den Of Geek, Eliza Taylor said that Clarke uses a routine to keep herself sane while it is just her and Madi for all those years. Each day, she does the same thing. Madi gets her lessons, they go hunting, and she tries to make contact with Bellamy Blake (Bob Morley).
Jason Rothenberg provided additional information, including about that small patch of green Earth. Even though radiation levels are down and the air is safe to breathe, most of the planet is a wasteland. However, that little piece of land is going to be a major focus of Season 5. When The 100 returns, others are going to want the land that Clarke discovered. This includes the people on the prison ship that are coming down from space. Clarke is not going to want to give it up, though. It seems that she might be prepared to fight for it.
"The prison ship has come down into it and there will be lots of people aboard that ship who want this garden for themselves, [but] it's Clarke home and she doesn't want to share it. And she and Madi's lives may not be what they have come to be used to. The battle for Eden is an ongoing threat this season."