Of the general options that exist in terms of hypothetical space launch systems, what is the most plausible option for a relatively near future SF setting? The focus here is on Earth to LEO, as the level of space development this cheaper access would imply also lowers costs by giving refueling infrastructure in orbit.
The few obvious ones that come to mind are:
Space elevators would be the best option overall in terms of lowering costs, but there is the question about whether we could actually pull this off in a practical sense. Besides the question of finding a strong enough material, a failure would be catastrophic in scope. It would also likely require literally perfect forecasting of lightning strikes, as the most likely material is carbon nanotubes that would be very conductive to electricity.
Laser launch to orbit works similarly to a space elevator in terms of offloading the mass of the fuel, but it trades the problems. While it would not require borderline impossible physics, it would require utterly massive energy storage or production at the launch site. It would also be an extremely potent weapon system that would make other nations that the owner nervous and possibly blind anyone looking at the launch.
Improving the fuel via metallic hydrogen is also a possibility, as probably the most high energy and high thrust rocket fuel that doesn't involve setting off nuclear weapons. It isn't the same level of extreme savings as something like a space elevator, but it could potentially lead to savings as it would allow proper mature SSTO designs with large payloads to develop. If fusion rockets could produce enough thrust to get off the ground, they would be also be an excellent option in roughly the same category, but that is not likely.
Reusable designs and their descendants. This is obviously what is happening already in terms of design, but even the most optimistic(and probably more than a little unrealistic) projections would not approach even the conservative projections of the more advanced concepts. One other possible related approach is that of the SABRE(or RAPIER for KSP players) engine in which it uses a ramjet/rocket hybrid, if that can ever be made to work.
Of these or any others I'm not familiar with, what is the most plausible general solution to give significantly cheaper access to orbit within the next century?