Shoujo Kakumei Utena, Darling in the FranXX Recommendations
While I feel that the RGU movie has more similarities with FranXX (which I've already recommended), I still feel like the tv series of RGU shares a good amount of similarities with FranXX that's worth mentioning.
As mentioned in my other recommendation, despite both being drastically different in terms of genera's and atmosphere—both share themes of sexuality, gender and relationships. While I feel the relationships and characters are different in the RGU movie vs tv series, the mc's we get to know through the tv series still share a lot of similarities between the mc's of FranXX—maybe even more so, especially nearing certain arcs. However,
I feel that RGU explores not just relationships but of one's self, as an individual, more than FranXX did. Another thing to keep in mind is that RGU is more queer focused than FranXX—though, it does delve into the subject matter a bit, but not by much.
As mentioned in my other recommendation, RGU feels like it was made for a more niche audience, while FranXX was made for a wider audience—in my opinion anyways. Because of that, I feel that one feels a bit more mature than the other, but I believe you have to look at them with different perspectives to see what the creators were trying to show. But if you started with RGU, you might find yourself having a harder time with FranXX, but I implore people to keep an open mind and to pay attention to the imagery, themes and symbolism of both—slowly, you'll start to see the similarities.
List of things they have in common (tl;dr)
~Themes of sexuality, gender, relationships
~Lots of beautiful imagery and symbolism
~MC's (and other characters) have a few things in common with one another
~RGU is more queer focused rather than FranXX but FranXX does touch up on the subject a tiny bit
~Both have MC's go against the status-quo to find their own truth and self
~Both have romantic themes to it that are done beautifully
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