There may not have to be, as far as we know. From the Wikipedia page[1]:
Many theoretical physicists believe these fundamental forces to be related and to become unified into a single force at very high energies on a minuscule scale, the Planck scale, but particle accelerators cannot produce the enormous energies required to experimentally probe this. Devising a common theoretical framework that would explain the relation between the forces in a single theory is perhaps the greatest goal of today's theoretical physicists. The weak and electromagnetic forces have already been unified with the electroweak theory of Sheldon Glashow, Abdus Salam, and Steven Weinberg for which they received the 1979 Nobel Prize in physics. Some physicists seek to unite the electroweak and strong fields within what is called a Grand Unified Theory (GUT). An even bigger challenge is to find a way to quantize the gravitational field, resulting in a theory of quantum gravity (QG) which would unite gravity in a common theoretical framework with the other three forces. Some theories, notably string theory, seek both QG and GUT within one framework, unifying all four fundamental interactions along with mass generation within a theory of everything (ToE).
Another related thing I've wondered is if, similar to how the possible electroweak force un-unified into the electromagnetic force and the weak force, is there a scenario where the electromagnetic force (or the weak force) un-unifies into some other sub force ? So I guess like the reverse of the GUT.
It is certainly possible. What we think of as a long timescale for the age of the universe may be very short in some future epoch where the fundamental forces further split, just as we see the epoch of electroweak unification as being short.
That isn't quite the same thing. Both electrostatic and magnetostatic interactions are mediated by the same boson (photons). Whereas each of the four fundamental forces have their own set of bosons (W and Z bosons for weak, gluons for strong, and the theorized graviton for gravity).
i thought about this as well, when i first learned about unification of forces, it seems like an obvious thing to try and reason about, so let's go!
[caveat - i'm not a physicist, but]
we know forces unify at higher temperatures/energies, and there is no limit to how high temperature can get (maybe? i guess temperature is the same as the mean velocity of the particles, so for particles with mass this cannot exceed c, right?) however the the high energy zone is inaccessible with current technology above a certain level.
but in the other direction, we hit a floor at zero kelvin. which we can access, and can therefore test. so we know that going right down to zero kelvin / electron-volts / joules / meteres per second does not split any of the known forces further. this is how we know that they are the fundamental forces, i think.
Many theoretical physicists believe these fundamental forces to be related and to become unified into a single force at very high energies on a minuscule scale, the Planck scale, but particle accelerators cannot produce the enormous energies required to experimentally probe this. Devising a common theoretical framework that would explain the relation between the forces in a single theory is perhaps the greatest goal of today's theoretical physicists. The weak and electromagnetic forces have already been unified with the electroweak theory of Sheldon Glashow, Abdus Salam, and Steven Weinberg for which they received the 1979 Nobel Prize in physics. Some physicists seek to unite the electroweak and strong fields within what is called a Grand Unified Theory (GUT). An even bigger challenge is to find a way to quantize the gravitational field, resulting in a theory of quantum gravity (QG) which would unite gravity in a common theoretical framework with the other three forces. Some theories, notably string theory, seek both QG and GUT within one framework, unifying all four fundamental interactions along with mass generation within a theory of everything (ToE).
[1] https://en.wikipedia.org/wiki/Fundamental_interaction