We can start looking at the infrastructure that makes moon or asteroid mining for fuel possible. 'Cheap' fuel in space is the dream because it means asteroid 'mining' for materials that get returned to earth can be profitable (massively profitable to the point of dwarfing any investment currently made).
But the amazing side effect is suddenly we also have materials in space that like the fuel now costs orders of magnitude less than if we were sending it up. It'll becomes viable to build more infrastructure in space and so (perhaps ironically) there will be more demand to get materials from earth to space.
Here's a writeup: https://exoplanets.nasa.gov/internal_resources/1375/
In general, once you get to LEO, you're half way to anywhere ... That is, getting to low earth orbit is half the battle. Spacex will remove that battle for everyone, making space expansion and exploration 2x easier.
My dream is earth-orbiting shipyards for outer planet missions and interstellar missions. There's nothing stopping us but will.
At 100t, that would be 400 a pop.
Well, 1 million people ain't going to get to Mars by themselves[1].
But once you have that sort of capacity, at those sorts of prices, many things that are now unthinkable become very possible and possibly useful.
Asteroid mining has been mentioned. Putting huge telescopes into orbit or the Lagrange points. How about a nice and big radio-telescope on the far side of the moon, where there is no EM interference. Or maybe even further out? How about some very, very long baseline-interferometry[2]?
Put manufacturing in space. There appear to be some useful materials that can only be made in low-gravity environments, but currently the Price is not Right™[3]. Bezos wants to put large space habitats into orbit instead [4]
Others have mentioned Starlink, with up to 12000 satellites. Nowadays they're talking about 42K satellites.[5]
Space-based solar power might become feasible[6]. Maybe put up a sunshade[7] to control global warming.
How about some manned deep-space probes? Maybe with some Vasimir rockets[8], a nuclear reactor and lots of reaction mass.
We are so conditioned to think that anything space has to be small, super-lightweight and super-high-performance/expensive that it's really kind of hard to think about the consequences of those constraints no longer applying.
And of course tourism, like the already-announced circumlunar flight[9], space habitats etc.
Think big!
[1] https://www.syfy.com/syfywire/elon-musk-mars-colonization (or just google it)
[2] https://en.wikipedia.org/wiki/Very-long-baseline_interferome...
[3] https://en.wikipedia.org/wiki/Space_manufacturing#Materials_...
[4] https://www.businessinsider.com/jeff-bezos-proposes-floating...
[5] https://observer.com/2019/10/spacex-elon-musk-starlink-satel...
[6] https://en.wikipedia.org/wiki/Space-based_solar_power
[7] https://en.wikipedia.org/wiki/Space_sunshade
[8] https://en.wikipedia.org/wiki/Variable_Specific_Impulse_Magn...
Obviously most of that mass would have to leave orbit in order for it not to end up looking like an LA freeway in rush hour.
edit: i guess propellant/fuel would be one of the heaviest and important payloads we would need to get up there.