The first in what will be a fairly long series (unless everyone really hates it I guess). As usual with twinning series there's not any particular theme to this rather its just guys turning into recognizable gals with whatever set-up charoset comes up with.
Kind of a weird one to start with honestly since the idea is pretty high concept. For those having a hard time following, the full idea here is that Luffy is looking to get rid of all the clothes Hancock gave him and deposits them in Nami and Robin's room. There he stumbles on Vivi's old Miss Wednesday outfit that I guess survived this whole time and triggers a transformation.
The concept being that Vivi's 'evil persona' as Miss Wednesday wasn't just acting but a full other personality that she hypnotized herself into and it works on Luffy because rubber/haki/tg logic, whatever you like.
Nami then stumbles in confused before realizing what happened and implying she and Vivi would invoke this personality for fun back in the day...
...Yeah it's a really over the top concept for a sequence. Future ones will be a bit more as you would expect, the guys being changed by the other person's power or gadget or whatever. This is technically last month's post in this series so watch out for another one this month with Zoro.
Lastly, I know twinning is for many people a love it/hate it type of transformation. I do have something I'll be adding later this month once I'm a bit more caught up that hopefully will satisfy some folks in the 'anti twin' camp.