One they can become the "FCC/FAA of space". The last A is "Administration" of course. This should not be considered depressing, as the scientists and engineers currently on the front line of NASA discoveries are already often associated with other institutions and could easily follow the trend.

Two, they can become the science arm or funding agency for space-related science. This is very close to their actual role. You don't see a ton of commercial submersible traffic, for example, and Earth should definitely be considered fully commercialized and infastructure-supported, and yet NASA still funds, designs, and operates missions for Earth science. So, there's no reason to think that NASA will somehow fade into the background for good. As I've often said, if someone "solves" launch and "solves" telcom over interplanetary distances and "solves" transport of humans and "solves" logistics at a dozen AU, then NASA can finally focus on just instruments, experiments, and science. It's like you've been building your own car and highway and computer every time you want to go to work, and someone comes along with commercial versions. You are more productive.

The third trajectory is liquidation. NASA can become a funding agency like DARPA or NSF, leaning on their past glory to inspire a new generation with fancy-titled grant calls for science experiments at scale.

I personally believe it's a convex combination, and my favorite weights are about 1/5, 3/5, 1/5, meaning they'll focus mostly on deployment of science instruments supported by commercially-designed platforms, technology, etc, and release calls and grants to other institutions to propose science experiments and develop non-profitable technologies that industry will necessarily ignore.

Coincidentally, this is about how they operate on Earth.