What are the minimum requirements for a new fundamental theory? If we knew a most-fundamental theory of physics, the many currently-known equations would be derived naturally from it.  Perhaps, most of the unsolved problems in physics may be fully explained by the most-fundamental theory. However, at this time, it seems difficult to expect that we will be able to achieve all of it.
So, more realistically, let's assume that we have just a new theory that is likely to be the most-fundamental theory but is in an immature state. That is, suppose that further studies are needed to know whether the new theory is the most-fundamental theory. In such a case, what are the minimum requirements for the new theory to be qualified as a candidate for the fundamental theory?
One example is given in this site, although, to me, requirements 5 and 6 therein seem excessive.)
(Please remember that I am asking what are "minimum" requirements for an "immature" theory.)
Thank you.
 A: All of the requirements in the example can be simplified to a single requirement. Then there is one further requirement:

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*It must explain all the observations done by previous experiments with fewer initial assumptions.


*It must predict at least one novel outcome of experiments that have not yet been performed, so that it may be falsified.
A: Theorists are looking for a theory of everything (TOE)
One reason string theory is popular with a number of bright young theorists is because it fulfills the following criteria that any such theory must do, in my opinion:

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*It can embed the standard model of particle physics in its group structures. The standard model is like a mathematical data bank of almost all the data gathered in the past hundred years on particle physics.


*It allows for the quantization of gravity, so that a unified theory can emerge.
The problem with string theories is that no particular one has been found, from the very many possible that have been proposed to be checked, in producing new predictions, that would validate it. Theorists keep working.
All other physics theories can be shown mathematically to be emergent from the underlying standard model quantum field theory.
For a new proposal for a fundamental theory, these two should be the minimum requirements, even in a hand waving way in the beginning. The standard model has to be there as it is really the data. Gravity might be a second level.
A: Well for a theory to be considered as a candidate for the new potential unified theory, it should provide some new "perspective" that the current theories do not have. Weak interaction, strong interaction and electromagnetism have been somewhat unified under quantum mechanics, the current challenge to be able to link these forces to gravitational force under the same model !!!!
But at any point of a new theory, I think that the new model should meet with experimental results in some "situation", and then physicists will have a reason to look further into the proposed model. Quantum mechanics started as a conjecture, but when it started explaining some phenomenons that classical mechanics couldn't, more physicists started to be more interested. I think the same kind of process should happen before calling the new theory a candidate for the unified theory. I think the challenge is modelling the gravitational force with the other under the same formalism.
This is my point of view, I don't think that all the physicists would agree with everything I've said.
A: Your new theory must derive the Lagrangian of the standard model of particle physics. This includes all the details.
And your new theory must derive the Lagrangian or the field equations of general relativity. With all the details.
So far, no theory does.
