I don't know how to design things but i think that good ideas would be body pillow with the main girls of the game. (Bunny, Juliette, Red Battler, Kalissa and my favourite Rabbi). What do you think about Harem Heroes little piggybanks? (the shape can be the MC dong or a girl with removable clothes sit on a rock (the first girl i have in mind is Mountain Sophie). I like the idea of the action figures with sizes between 15cm and 50cm with real cloth clothes. The figures can be static and in one piece (small ones) and with joints with real clothes (bigger ones) I think would be good a small set of the city (like Lego sets) and all the worlds would be expansions of the main city and every world have the enemy and his girls and the story girl. I know we are all adults here but i think it's not a childish request this one.
My last two suggestions are: a comic with all the scenes of the game (and some inedited stories) and an anime.
I hope my ideas will be of some use. I love Rabbi...