Fusuma Land 4.5 by Waki Yamato 1 volume -complete
Before there was Platform 9 ¾, there was Fusuma Land 4.5. Kachiko is a very strict, glasses-wearing and a short girl. One day she fell into Fusuma Land where you transform into someone or something you wish to be. She also meets a ragtag of neko and literary/legendary figures.
Before you judge the manga by it's art, please take note that this is from 1984 and probably one of the pioneers of reverse harem as we know today. The story is original and all the elements are neatly planned out and paced well.
I've also updated Intro to Reverse Harem and Site with the pioneers of reverse harem. As per Mangaupdates, the oldest reverse harem mangas that we have are "Soredemo Chikyuu wa Mawatteru" (1984), "Fusuma Land 4.5" (1984), "Akogare Boukensha" (1985), "Momoka Typhoon" (1987) and "Hanasakeru Seishounen" (1987). They should also add Hiatari Ryoukou! (1980).