Oh, my man...where to begin. I played most of the FMG games, and honestly it's up to you what you like.
I noticed a patern also with people liking main characters that start of frail skinny weak, helpless bla bla you know and then as they gain muscle and strength their personality changes more on a dominant side.
Also start of with a small open story that you will slowly expand through updates, for example take Sarah, a girl you met and the library picking up dumbells and you go to the gym with her to help her train, so she doesnt become immediately buff, you are with her through the process of it and you both love it...(pervy scene right there).
Also with you meeting new people, rivals, enemies etc.
It's your world play with it, open up a patreon and ask the subscribers what they want aswell(for me futa option would be neat)
Also change the description of the game a bit, give a small intro when you open itch.io
And play some other fmg games to find more inspiration.
You already got the skills to make a good game now all you need is to release that pervy mind!
Good luck!