We've made great progress already in more or less completely understanding baryons (composite particles with three valence quarks or three valence antiquarks bound by gluons), pseudoscalar mesons (composite particles with a valence quark and a valence antiquark of a different flavor bound by gluons that have spin-0 and odd parity), and vector mesons (composite particles with a valence quark and a valence antiquark of a different flavor bound by gluons that have spin-1 and odd parity).
We have now crossed the threshold into understanding excited hadrons (i.e. higher energy states of other hadrons), quarkonia (composite particles with a valence quark and a valence antiquark of the same type bound by gluons, which are mesons), "molecules" of two mesons, a meson and a baryon, or two baryons that are bound by residual strong force interactions (much like protons and neutrons bound in an atomic nucleus), true tetraquarks, pentequarks, and hexasquarks (systems of four, five or six valence quarks bound directly to each other by gluons rather than in a hadron "molecule"), and blended states of hadrons that are bosons (composite particles with even numbers of valence quarks) including blended states that include glueballs (composite particles made up of gluons without valence quarks).
This broader understanding is leading to progress in understanding the true structure of scalar mesons (i.e. spin-0, even parity) and axial vector mesons (i.e. mesons with spin-1 and even parity) mesons, and other exotic resonances.
In the "medium-long" term time frame (i.e. 40-60 years), I'm reasonably comfortable that all of these issues will be more or less completely worked out in principle. We will have determined every theoretically possible hadron resonance, described its structure, and experimentally confirmed its properties. Even 40-60 years from now, however, I suspect that the experimental measurements of these properties will be more precise than those calculated from first principles.
I'm also cautiously optimistic that great progress will be made in calculating parton distribution functions (i.e. a probabilistic description of which fundamental particles with which momentums are found within a hadron) from first principles rather than merely through empirical trial and error, and in improving the precision with which we know the Standard Model physics constants pertinent to quantum chromodynamics (i.e. the Standard Model physics of the strong force) such as the strong force coupling constant and the quark masses to at least one or two orders of magnitude more precision. We will also make progress in doing calculations with QCD with more precision and less of a computational burden.
The existing Standard Model laws of physics applicable to hadron physics are already known, in principle at least, exactly and completely, although operationalizing these laws of physics in a manner that we can do meaningful calculations with is an ongoing process.
For the most part, we don't need any huge break throughs, although I wouldn't rule out the possibility that the existing QCD laws of physics are supplemented in the next half century with one or two or a few subtle additional rules or principles (e.g. a discovery that there is some principled reason that there can be no pure, free glueballs, only of the earliest predicted hadrons which is never been observed).
As I noted in my previous post, the heaviest observed hadron resonance that is well characterized to date is just about 6 GeV and the theoretical maximum mass of a heavy hadron resonance is in the low tens of GeVs. So, we don't need particle accelerators vastly more powerful than those we already have to make great progress in this area.
The Physics Desert, Grand Unification, Theories Of Everything And Within The Standard Model Physics
I strongly suspect that we will see a "desert" or "nightmare scenario" over the next 40-60 years in which we see no experimental evidence of beyond the Standard Model physics (apart from whatever resolution is found of the dark matter and dark energy question and of the nature of neutrino mass), and as noted above, that the anomalies that we do see will disappear.
Quantum gravity, when it is figured out, will tweak the beta functions of all other renormalizable Standard Model physical constants somewhat, which may or may not shed new insights on our theoretical expectations regarding the extremely high energy behavior of the Standard Model at energy scales which will not be reached anytime in the next 40-60 years by experiments or astronomy observations.
I think that it is increasingly clear that the masses of the fundamental particles, the CKM matrix, and the PMNS matrix, which account for 23 of the experimentally measured constants in the Standard Model are fundamentally electroweak phenomena, as are, by definition, the electromagnetic and weak force coupling constant. Thus, 25 of the experimentally measured constants of the Standard Model are electroweak phenomena, while only one of the Standard Model physical constants (the strong force coupling constant) is not an electroweak phenomenon (excluding Planck's constant and the speed of light in a vacuum).
It is also almost inevitable that the measurements of every single one of those physical constants (and also of the three Standard Model coupling constants and the gravitational coupling constant) will improve significantly in the next 40--60 years.
I am cautiously optimistic that the electroweak sector of the Standard Model, informed by increasingly precise measurements of its experimentally measured physical constants, will be explained from far fewer true physical constants with a further elaboration of unified electroweak phenomena.
Hints like (1) Koide's rule, (2) the approximate accuracy of extension of Koide's rule to quarks and patterns in this extension's errors, (3) the possible accuracy of neutrino sector extension of Koide's rule, (4) the fact that all particles that interact via the weak interaction have mass while all particles that don't interact via the weak interaction do not, (5) the fact that the sum of the square of the masses of the fundamental particles is equal to the Higgs vacuum expectation value squared (which in turn is a function of the electroweak coupling constant and the W boson mass), and (6) the patterns in the numbers we see more generally in the physical constants that we have measured (e.g. in the CKM matrix and in its correlations with the quark masses and parallel trends with neutrino masses and the PMNS matrix), all point to the possibility that the answer is out there and might be possible for a single genius to solve in one fell swoop and reduct the number of experimentally measured Standard Model physical constants from 26 to perhaps as few as 6 to 14.
Beyond development of a theory of quantum gravity and a reduction in the number of Standard Model physical constants with an expansion of electroweak theory, however, I am not very hopeful that there will be any progress towards Grand Unification of the Standard Model forces, or a "Theory of Everything" in the next half century.
We may rule out a lot of potential solutions with new experimental data and the quantum gravity progress described above. But I don't think that we will affirmatively find one.
On the other hand, I do think that there is an outside chance that once our scientific community accepts that there is almost nothing beyond the Standard Model to discover, and finds a gravitational explanation of dark matter and dark energy, thereby eliminating the incentive to create beyond the Standard Model physics to explain these phenomena, that progress could be made with a community that has spent the last half century going down the wrong fork in the road, that would now be refocused in the right direction in a much more highly constrained fashion.