To LOVE-Ru, Sora no Otoshimono Recommendations
Both animes have a female alien coming from out of outer space witch stays with the main protagonist. They also both have amazing technology.
It seems that both animes copied each other. Both anime containing a main character who has big boobs, is from outer space, and live to make the main male character happy, only to ruin his life. Also, the main male characters in both animes can be described as indecisive and extremely ignorant, which sometimes makes you think "why are you even with him?" Bottom line they are harems, sexy, and funny.
Similar origin of female protagonists. Both have super powers to grant wishes of male protagonists. Both are ecchi series.
In both animes, you can find one (super lucky?) man surrounded by alien beauties. Though there are some differences in the main male character personalities, both series have harem-based plot with some ecchi scenes as bonuses.
Lots of fan service
MC builds a Harem
Very similar Ecchi and Harem series. Both have an indecisive main character (Tomoki / Rito). Both have a busty girl who comes to "ruin" their lives (Ikaros / Lala), and both fall in love with the main character. Both series mix "religious" and supernatural thematic (Angeloid= Angel Droid / Deviluke= Devil Alien). Both series involves a conflict against an evil powerful leader (Master of Synapse / Gid Lucion Deviluke), who wants back to the main female character in her respective place of origin.
To me, both series are perfect for a crossover.
-Guy who meets girls from an another planet
This is pretty much the same. fill up episodes lots of ecchi. female alien comes to earth and starting an harem. boy is Perverted and has a lot of different friends with different interests. Pretty much these two animes are Twins
And...you have to watch it :D
Both are quality harem Sci-fi with one boy involved into bunch of many girls