 If you liked
Ima, Soko ni Iru Boku
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 ...then you might like
Brigadoon: Marin to Melan
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Both have quite serious story disguised behind animation style you would't expect, similar main character hardships upon discovering something/someone from another dimension and travel between them, sci-fi and fantasy elements. Ima, Soko ni Iru Boku more tragic in usual context of tragedy, while Brigadoon is more of a tragicomedy, depicting a lot of serious issues trough jokes. You will definetly like Brigadoon if you liked Ima, Soko.
Both shows present us with young protagonist, otherworldly realms and sci-fi setting. Although Brigadoon does use comedy, while Ima, Soko ni Iru Boku doesn't, both shows do have saddening and tragic happenings in their plots, and the mood is quite serious and somber. Also both shows do use their artstyle as a deception, looking kid-friendly and childish, while being shows with alot of dark moments.