We are the opposite, my hubby says he has no fantasies (anymore, he told me about a couple and we did them) but I frequently have new ones.
You could try watching some of the saucy end of programmes together (eg sex/life, 365 days, 50 shades) and then see if they bring up any discussions (away from the bedroom).
Or bring up that you’ve seen a thread on fantasies and discuss a few you’ve seen there? She might just not be able to explain what she wants so hearing about others may prompt her.
Edit - but as always, if she says no and isn’t open for a discussion about fantasies, don’t push her, it may be a genuine she just likes straight sex!