It surely can't be that long before Rockstar announces a new GTA game now. Once they have got the release of Red Dead Redemption 2 out of the way, I suspect it will just be a matter of time.
A female protagonist is certainly an interesting concept. After the enormous success of GTA 5, Rockstar are going to have a tough task creating a sequel that lives up to it's predecessor. Perhaps a female protagonist is just the kind of thing the series needs to take it in an exciting new direction. It's not really something that's been done before (the PC version of the original GTA allowed you to choose from several characters including some women, but that I'm not counting that). I think if done well, it could really work.
Most Helpful Girl
I certainly think if they keep the 3 protag idea from GTAV, that a lot of people liked, having a female character would be neat.
They used Trevor to explore drugs and arms dealing, Franklin for vehicles and racing and Michael had more of an emphasis on juggling his family life.
With this is mind, I think having a female protag explore something like prostitution or something might be really cool.