Now, before I answer your question, I will say that Imu has hightly possubility to be girl and final villain, so strongest character in OP world could be already female character(I mean she love to spend all day in garden and made 6 man to rule world, while she doing nothing , and only sometimes give crazy orders about commiting genocide and erase history sheet).
This whole topic is bit wrong case creating new female character is not that simple, I could show some reason why Oda is doing this:
1) Storywise it would be hard to justify strong female character who is still looking like female, now I mean with thick body, nice breasts, and just a bit strong arms. Imagine MissMonday that is even bigger, would you be satisfied? I doubt. While we already saw strongest male characters and all of them looking like bodybuilders who used too much testosteron: Garp, Akainu, Kaido etc
2) Storywise it would be hars to justify strong female character: girl should train 24/7 or fight all time, getting a lot of scars etc. So literaly you need girl who acting like a man...
3) You shoud explane why this big breast thick girl is Stonger than Luffy or Zoro...maybe bigger ambitions?!
Now ofcourse there are some ways of creating beatiful strong females:
1) Special creature - that Yamato, she was raised like a man, case I guess that Kaido idea to call his daugther as son. So we got good genes and powerful girl, who have everything that male category wanted to see, except her personality, which could be changed in the end of arc;
2) Science Experiment - that's Reiju who is actually strong enough, with problem that she didn't justify killing people, which made her weaker;
3) Moonlight mink girls- they got power only when Moon is full;
Now I want to discuss BigMom and why Oda doing that things with her. I believe that Oda just wanted to made BM gag character like Buggy, that made people to laugh. Why? Maybe that his way to somehow justify her, showing that she is not like Kaido. Oda even turned her to O-Lin, so I see that he wanted readers somehow like this character, just to made death of BigMom a bit different than Kaido. Which I doubt that could work, but I really believe that is Oda way to justify chatacter, and it's not cased by his woman hate.
So now lets talk about SH girls Nami and Robin, yes fanservice is main problem, but there are others:
1. Nami - storywise it's hard to give haki power to her, case even in timeskip she just learned science, and there wasn't showed any opponent that she could faced there. Ofcourse it could work if she awakened haki in battle now but for that she need to get full fight 1x1 which should happened in WCI arc and it never happened there. Instead of that Oda gave het Zeus power up, which actually could work for her, but only time will show how that could became permanent, case I don't see any other legit way for her to keep Zeus, except only eating devilfruit, which is different topic of discussion.
2 . Robin - so far I just don't understand Oda and why he never let her fight...she could have haki, and she is already strong...
P.S. In the end I blame editors for all fancervice that happened in OP, especially with Nami and Robin, I loved One Piece before timeskip so much, idk why Oda/editors decided to turn OP fanservice(which was here) into that really dirty staff, that made me sick, like bath in Wano with Nami and Robin.