Muu no Hakugei

Both deal with reincarnation and finding out one's past lives in a group, dealing with the different conflicts that arises. Even if Muu is much more of a battle shounen it has still quite a relational and emotional focus. 

White Whale of Mu is like a spiritual precursor to Fantastic Children. There's a lot of similar themes and symbolism like ancient technology, reincarnation, multiple timelines and so forth. 

They're both about a race called Mu... and they both have a ship called "Moby Dick"... ...wait a minute!