NieA Under 7

both r by the same creator....and the art,character design and atmosphere is pretty much the same... 

same art style and same atmosphere. both are a little mysterious because Haibane Renmei deals with angels while Niea Under 7 deals with aliens. Both also deal with human behavior and such. Niea Under 7 is like the comedy and less serious version of Haibane Renmei in my opinion.  

Both are slice-of-life anime that have their lighthearted moments, but also feature some darker more cerebral, and introspective moments. Both feature characters were designed and created by Yoshitoshi ABe, who also did the artwork for Lain and Texhnolyze as well. Both are more mellow, chill shows with worlds that have creative lore behind them. Both feature similar story and character beats but do them differently enough to differentiate themselves from each other. Both also feature heavy acoustic and softer soundtracks to accommodate their vibes. Both shows are also just wonderful in my eyes too, so yeah. 

Both shows are over 15 years old now and have slice of life elements mixed with a sci-fi kind of setting. Black Heaven is mainly a music show while NieA_7 is mainly just lighthearted slice of life. (They're also both better than their ratings advertise) 

both are downbeat sci fi themed stories that can easily be completed in a day. 

Both have a ya cast living together and a focus on their friendships despite what strange differences that they may have and also some pretty cheeky comedy as well too.  

Strange girl of mysterious species shows up and becomes roommates with human main character, who is shocked at her antics. Slice of life hijinks ensue 

Both revolve around human and alien girl, Yuri and Niea have really simmilar designs, both are slice of life comedies 

both are sci fi themed stories with a diverse cast and also staring an eccentric curious young girl in it as well. 

Both are about a woman whose home is invaded by a supernatural creature like dragons or aliens, changing their lives completely. Kobayashi goes more the comedy way while NieA_7 is more of an iyashikei, but both have similar premises.