 If you liked
Koukyoushihen Eureka Seven
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 ...then you might like
Kiseijuu: Sei no Kakuritsu
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Although appearance wise they may have nothing in common- one about parasytes and the other being a mecha and such, the first thing that came to my mind when finishing Parasyte was Eureka Seven. They have strikingly similar themes.
They both deal with Earth and nature and the bond between two species. Their endings are pretty similar as well ^^