If the anime has several heroine routes then it's probably for the best to not adapt it unless those routes are very different like Fate/Stay Night or Tsukihime. If you're going to adapt a VN pick one with a linear plot. You cannot "merge" routes without fucking it up, it simply cannot be done.

The other huge fuckup is taking a legitimate story with porn and making an adaption that is focused on the porn. Tsui no Sora is a great read but the adaption is fucking horrendously unfappable hentai, YU-NO is considered a masterpiece and it of course focused on the porn. VenusBlood and Rance are amazing games with really fun stories (especially VB Hollow, holy shit that goes completely insane in the Chaos route) but the adaptions, again, focus entirely on the porn which doesn't do it any favors.

Anyway the point I'm trying to make is fucking adapt Sekien no Inganock.