Supergirl ended its third season on the kind of cliffhanger that can drive fans crazy over a hiatus while they wait to find out what happens next. The final scene revealed a Kara doppelgänger, created after using the Rock of Yuda Kal to take down Reign, walking toward a base in the snow in Siberia. We learned right then and there that Season 4 is going to include Supergirl as well Supergirl 2.0 in an adaptation of the unforgettable "Red Son" story from DC Comics lore. Melissa Benoist spoke at the recent San Diego Comic-Con about how Supergirl will handle "Red Son," saying this:
I think that -- the doppelgänger that we saw at the end of Season 3 is gonna be a really cool experiment in 'nature vs. nurture,' I think, in the way that 'Red Son,' the Superman comic, explored it. Showing how two people with the same abilities can be so different, and what kind of heroes they would be, and what her ideals and morals and moral compass would be. So I think that's going to be a really interesting thing to explore this season.
Kara became the person that she is in the present on Supergirl due to the fact that she was taken in by a friendly family when she landed on Earth, getting a foster mom and dad as well as a beloved sister after losing everything she knew and discovering her baby cousin all grown up. She learned to live without using her powers and carry on in a way that let her fit in with humans. Kara 2.0 won't have the same background that created an empathetic and understanding woman who feels driven to fight for good the way Kara does.
A big question is whether the lack of Kara's upbringing will result in Kara 2.0 the villain or Kara 2.0 the person looking for a home and identity of her own. All we saw of the doppelgänger in the Season 3 finale was her striding out of the snow, wrapped in a sheet and looking confidently around the Russian base she happened upon. Is she a fundamentally good person who wants to fight for good, like Kara? Or will she be molded into a weapon for use by the Russian government?
Supergirl has raised the point before with Superman's general unwillingness to work with the military that a Kryptonian whose interests are aligned with a government has the potential to become very dangerous. If Kara 2.0 is more of a product of nurture by the Russian military than an example of nature with a good heart and optimistic outlook, the entire population of the planet could be in danger.
All things considered, it sounds like a major theme of Supergirl Season 4 will be duality, perhaps even more than Season 3 with Reign/Sam and Supergirl/Kara. Season 4 could force the Girl of Steel to deal with fear vs. hope (as with Agent Liberty), hero vs. CatCo (a la the new trans superhero), the U.S.A. vs. Russia, aliens welcome on Earth vs. aliens decidedly unwelcome on Earth, etc.