So in a way they're already charging for (earlier) access to fast pass.
If they want to, it'll be pretty easy to introduce more ways of upcharging and then claim it's because of popular demand and serving people better.
If they introduce gold day admission tickets it's simple to add gold+ with 6 FP+s to "give you more options ... more ways to enjoy the parks! [TM]"
That or a thousand other possible ways of adding FP upcharges... If they're thinking along those lines ...
Not predicting it, just acknowledging the possibility.