Hello!
I have cosplayed sexualizing characters before at Fanime and had to ask staff Incase it was indecent (it wasn't. It was a bikini) but this year I'm going as Do-s Monster Princess from One punch Man. As everyone who has watched it knows, she had JUST a tiny bra and bondage straps. I plan on making the bra part larger to cover more breast area and the underwear will be bikini type which will cover all parts but I am not sure if that is still indecent? The bondage straps already highlight the entire cosplay and I really don't want to be told I can't be there because of her fit.
(https://vignette.wikia.nocookie.net/onepunchman/images/c/c0/SlingshotSFull.png/revision/latest?cb=20160227112022)
I would imagine that there are two possible options:
a) Wearing some kind of skin colored bodysuit underneath
b) Wearing the costume during the evening when there are less children around
Though since you did mention that you are making the bra part larger and altering the bottom, I think you might be okay either way without going into either option.
I talked to Fanime staff! They said it's fine as long as it looks something like a bathing suit.