A really fascinating scenario is this:

Let's say something like Libra gets adopted worldwide. Let's say there's a huge financial crash at some point in the future, with lots of currencies, including some in the Libra basket (the stuff pegging it to the real world), going off the rails.

What would Libra's powers-that-be do then? They can tweak the parameters of their system any way they like. They could throw any currency out of the basket. Or do whatever they like.