Greta Van Susteren is a very talented newsperson, and has great interviewing skills. She is able to dissect situations and explain them so that people understand them. At Fox she was forced into fitting into their political agenda, and leaving means she can spread her wings and show off her skills.
Yes, leaving Fox News will lead to better opportunities for Greta Van Susteren. Throughout 2016, women have been in the spotlight at Fox News, particularly as victims of sexual assault. It has become a toxic work environment for female employees and it is in everyone's best interest to jump ship before long.
She was one of the "top" anchors on Fox for the past decade. I think if you compare the type of reporting on Fox to that on a more legitimate news program, she won't really have better opportunities. Plus, although she planned to leave, it seems that Murdoch hastened her departure by removing her from the air after he was displeased with her attempt to re-negotiate her contract. I don't think another network will offer her the same kind of money, and I don't see her back on CNN.
No matter where Greta ends up, it's got to be better than the conservative bubble that is Fox News. Hopefully she'll find a program that lets her do some actual investigating and utilizing her talent. She was wasted at Fox as just another one of the attractive anchor ladies. I expect big things from her in the future.