If you loved RahXephon: Pluralitas Concentio, try Psalms of Planets Eureka Seven: Good Night, Sleep Tight, Young Lovers
A bridge between a film you've already seen and one most people haven't. Here's what they share, and what the second one does that the first one doesn't.
You already loved

RahXephon: Pluralitas Concentio
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Psalms of Planets Eureka Seven: Good Night, Sleep Tight, Young Lovers
What they share
Both films are directed by Tomoki Kyoda, and they both carry the bittersweet mood tag, and they sit in Animation / Science Fiction territory. If that's the register that drew you to RahXephon: Pluralitas Concentio, the second film will land in a comparable space — through a different lens.
bittersweet
What Psalms of Planets Eureka Seven: Good Night, Sleep Tight, Young Lovers is
Apparently someone thought mechs and romance mixed. Renton joins a military unit to rescue Eureka. It gets complicated, naturally.