According to Variety, The 100 season seven will include a backdoor pilot episode (the industry term for when an episode of an already-existing TV show does double duty as a regular episode and a concept for a spin-off show). The episode and potential series will be set 97 years prior to the events of The 100, shortly after the nuclear apocalypse that ended most life on Earth and sent a portion of humanity into space to wait out the nuclear fallout for the next century.
The 100 prequel series would follow the people on Earth who survived the apocalypse, following “a band of survivors on the ground as they learn to cope in a dangerous world while fighting to create a new and better society from the ashes of what came before.” This group of people are the ancestors of the group of people known as The Grounders on The 100.
Jason Rothenberg, who developed The 100 series from the young adult book series of the same name by Kass Morgan, will also be developing the prequel series, alongside Leslie Morgenstein and Gina Girolamo. The CW also has a planned backdoor pilot for The Canaries, which would be an Arrow spin-off should it go to series.
Both Arrow and The 100 are in their final seasons this current TV season (Arrow season eight having just premiered and The 100 season seven set to premiere in 2020). The 100 pilot premiered in 2014, with Arrow premiering in 2012. It will be strange to have a The CW season without them for the 2020-2021 season, but would be fitting for their legacy to live on in spin-off shows.
The 100 airs on E4 here in the UK.