3x3 Eyes should have been a whole series.
The author shouldn't have opted for an early OAV and should have heald out for making a series once his Manga got popular. Which it did. But I guess he doesn't really care anout the Anime series since his Manga was one of those that went on forever and originally, people thought his Manga about 'monsters and demons' would 'never sell' in the first place.
Still, why do good Mangas like 3x3 Eyes, Hajime no Ippo and Ah! Megami-sama have these small Anime adaptations when other long Manga have almost all of their Manga put out into Anime form and even twice over in filler form...
I guess the lesson is, you just have to write nothing but Shonen.