I think it all depends on her contract with Disney and Marvel Studios.

I think it also depends on what she and Feige have discussed regarding the character. According to Tessa Thompson, an All-female team up was discussed with him. If they don't do a BW solo, I could see Scarlett headlining that instead.

I also wouldn't be surprised if she wants a break. I noticed that Isle of Dogs and Avengers: Infinity War aside she really doesn't have anything planned for next year and she only had GitS and Rough Night (which was a small production) out this year. She got divorced too and has a kid so she may want a lighter load.

Marvel really screwed up not doing a BW movie. They had a bankable actress, with a popular character in a popular franchise and they didn't utilize that. Lucy had a budget of $40 mil., BW would have had around the same production value. So the movie was never really going to be a loss for them.