A similar foundational cornerstone of physics is the principle of rotational invariance. Suppose that the laboratory finds that neutrinos (or anything else) have different oscillation rates when going in the N-S direction than in the E-W directions, in a vacuum, with no relation to anything else. This would break physics just as badly as faster-than-light neutrinos. If a laboratory announced this result, they would be laughed at, but the faster than light neutrinos are the same thing with respect to space-time.
There are no published tests of rotational invariance which are as good as the tests of relativity, partly because rotational violation is counterintuitive so no-one bothers. But for a modern physicist, relativity violations are counterintuitive in the exact same way.
A second foundational cornerstone is translation invariance. This is the principle that there is no way to tell where you are in an absolute sense, without measuring relative to something else that's there. If we found a magic spot--- a position where muons didn't decay for example, and this spot was just somewhere, you couldn't get it to move, this would be a violation of translation invariance. Translation invariance is even more fundamental than rotational invariance.
The experiments which would show violations of these are:
- violations of momentum conservation law
- violation of energy conservation
- violation of angular momentum conservation
- violations of the law of center-of-mass motion.
- violations of CPT (matter/antimatter symmetry)
If the Neutrino observations hold, they lead to a violation of center-of-mass conservation law for sure. You can move something's center of mass in one direction without emitting anything, just by propagating superluminal neutrinos one way, converting them to photons, and sending the photons back the other way.
Other than the space-time symmetries, the other inviolable basic principle is quantum mechanics. If you find a particle whose position and momentum are not uncertain, or which is not described by probability amplitudes, then you break quantum mechanics. This is difficult to imagine, because if one part of the world can be superposed, it is difficult to see how another part doesn't get the superposition by interacting with the first part. But the principles of quantum mechanics allow a deformation with decoherence, and this gives the Lindblatt formalism for density matrices. So a violation of Quantum mechanics is usually thought of as a certain amount of irreversible decoherence
- No irreversible decoherence in fundamental systems
These are the main experimental facts on which modern physics is built which could not be accomodated easily by modern physics. The first four are pretty sure, but Hawking tried to get irreversible decoherence in black hole physics as recently as 10 years ago.
If you put these together, there are relatively plausibile deductive paths that lead to relativistic field theories that are used today. If you add some assumptions both of a gravitational and non-gravitational nature, you find string theory should be the correct gravity theory. There is nothing below string theory, so you are done in terms of fundamental theories.