What if particle colliders cannot find anything beyond the standard model? What if the LHC and further colliders can not find anything beyond the Standard Model?
Nightmare scenario:

*

*LHC can not find anything beyond the SM-Higgs-like boson.

*VLHC, the linear collider or the muon collider can not find anything beyond the SM-Higgs-like boson.

*The 100TeV-1000TeV collider, likely in China, can not find anything beyond the SM-Higgs-like boson.

Then, what to do if no hints of dark matter particles, the axion or alike, new particles, superstrings, extra dimensions or anything beyond the SM-Higgs-like boson?
What if the nightmare scenario does happen? I mean the following: even if the SM holds to a very high energy, we should guess other ways to test its validity outside the collider realm or Dark Matter detectors. So, I believe it is NOT an opinion based question to ask what to do if the SM is unbreakable and nothing new arises in the next colliders.
Remark: I don't understand WHY you are closed this question. It is NOT an opinion based issue to point out WAYS to direct physics IF collider physics can not provide answers to fundamental physics questions like why the Higgs has a 125GeV mass? What is dark matter/dark energy? If the responses to some of these and others is not accessible with colliders, we should guess other ways, and that is NOT opinion based answers!
 A: If all particle physics data are consistent with the standard model (and inconsistent with various low-energy extensions of the standard model), then that will be an empirical fact about the world, that is to be taken into account when aiming for the deepest understanding that we can.
Theorists can still try to explain the parameters and structure of the standard model, for example. And in fact there would remain opportunity for prediction here, in that we only know those parameters to a finite degree of precision. If someone has a framework that predicts specific values for those parameters (e.g. a particular string theory vacuum), then improved measurements will make it possible to scientifically test their framework.
Cosmology and astrophysics also offer opportunities for new predictions to be made and tested, regarding energy ranges far beyond what a terrestrial collider can achieve.
Concretely, if nothing beyond-standard-model turns up, there are many things, both empirical and theoretical, that still need explaining: neutrino masses, dark matter (or the astrophysical and cosmological phenomena that are usually explained in terms of dark matter), the cosmological "lithium problem", cosmic baryon asymmetry, PeV cosmic neutrinos, the "hyperon problem" in neutron stars, why the Higgs mass doesn't receive large contributions from quantum-gravitational states.
A: We will do what is always done in science (when we come up short).  We will look for new approaches.  There are, perhaps, other ways to understand the fundamental particles/interactions of nature other than smashing particles together.  (And, perhaps even methods that might be cheaper and less energy intensive than colliders.)
A: As we do now we will try to look for other experiments to test fundamental theories (like precision measurements of the dipole moments, search for proton decay, search for deviations from the newton law etc that we use now to constraint various ideas about beyond standard model physics), look for hints from the cosmic rays and astronomical observations and try to make even more powerful colliders possible.
And (as you mention testing superstrings) we know that at the Planck scale you MUST learn something new. So if we don't stop improving our experiments we will find the new physics sometime in the future (though if it appears only on ghe Planck scale it will likely be rather far future)
A: That would be the time to start studying theoretical physics because the legacy paradigms would have run their course and would have lost their explanatory power.
A paradigm shift would be imminent, and you could be part of it. The situation would resemble the second half of the 19th century when there seemed to be a consensus that the entirety of physics had been laid out and only details were left to fill in. Wasn't it Planck (thanks, lalala) who was told not to study physics because there was nothing of importance left to discover?
A: We learn things from "failed" experiments, too. For instance, there are a number of theories about what dark matter is made of, and they posit different energy levels that would be required to reveal them. As we increase the power of particle accelerators, the theories that say that something should appear at that level get ruled out, and we narrow down the possibilities.
If we succeed in eliminating all theories, it means we've been thinking about this all wrong to begin with, so we try to come up with new theories and hope that we can develop experiments to test them. This could be the "paradigm shift" mentioned in another answer.
