For thous who do not read manga or watch anime.....DO IT I recommend an idea based off of Shina Dark a fun herem manga that is simler to 1000 virgin mars and one badass overdude but with a charter that fits more of the Daemon king personality aka Spike . AND it allows for really good monster girl set ups with all the hero and Demon lord stuff

ps heres the link http://www.mangahere.co/manga/shina_dark/

Sorry I am just not a big fan of harem animes.