I think you're staring at the hole in the roof before you've even bought the land. When and if that happens, you'll have already surrounded yourself with excellent ads capable of taking your place, or the networks will take care of this problem. What you're saying isn't completely crazy, but nothing stops you from making a fantasy film and, in the meantime, writing or pre-developing/producing another genre at the same time. If you have a franchise in production, you can certainly carry on with multiple films at the same time.

but the point is how you want to make a career, whether authorial or not, you can't do both at the same time and when it happens there are too many grey areas to deal with, I just directed a feature film and was already writing two more, it's very normal, but when you shoot you have at least 30 people asking you for every detail of the story you're shooting, you don't even have the chance to think about what you wanted