Series: "Magical Trans!"
https://www.mangaupdates.com/series.html?id=135400
http://preview.tinyurl.com/huc9x3x

(Guy joins a magical girl club.) I think it started with this sketch:
http://www.pixiv.net/member_illust.php?mode=medium&illust_id=28694691

The funnest part about being a magical girl would have to be jumping on buildings and soaring through the sky, even if there wasn't any tg, or monsters to fight.

Turning into a miko would be fun too, since they get to deal with unseen mythological creatures.