how about Girls from another of the games on Nutaku? Hold an event every three months or so that focuses on a different Nutaku game and its girls.

So an event that focuses on Flower knight Girls but done in HH style. Then a couple months later have an event that focuses on girls from Pussy Saga in HH style. Then a couple months later after that an event focusing on girls from Kahime Project done in HH style. Etc, etc, etc. The girls chosen from those games could be a combo of the devs favorites and players selection through polls.

Maybe also pair these events with cross overs with those games. ie play those games and do x, y, z and get kobans, another girl for the harem, affection gifts. Play HH and get things for those games as well.