> I am fascinated by the non-unification of gravity with other 3 fields of interaction.
Some particles don't feel the strong force, some don't interact electromagnetically, or weakly. But they all interact gravitationally. The three forces of the Standard Model are based on the degrees of freedom of "internal" spaces - the fibers of the gauge bundles. Gravity is based on spacetime dimensions. They are distinct in nature. They are united by the fact that the fields' Lagrangians have energy-momentum tensors, and these are, via Einstein's equation, sources of gravity. There is no need to try to unify the Standard Model forces with gravity in the same way electromagnetic force was unified with the weak one. Gravity's source is the energy, and electro-weak and strong fields have energy, like all matter fields. This is the way they are united: all fields are sources of gravity. I see no reason to try to unify the field with its energy-momentum more than they are unified by the relation
field -> Lagrangian -> energy-momentum -> gravity
Well, one good reason seems to be that the fields are quantized, and gravity not. But if we use the partition field approach to quantization, the quantization (partitionability) is inherited from fields to energy-momentum, hence to gravity.
If we insist to treat in the same manner all the four forces, we can consider the Standard Model fiber bundle as a manifold itself, as in the Kaluza-Klein theory, with some constrains imposed to the extra dimensions, and obtain thus a unification between gravity and other forces. But then, we need to explain 1. the constrains, 2. the relation given by the Einstein's equation between the matter and gravity, which now becomes artificial, being a relation between distinct components of the metric. There are also other approaches, based in general on the idea of identifying the Lorentz group, as well as the Standard Model group, with subgroups of a larger simple group.
But I think that it would be forced to treat identical gravity and the three Standard Model forces.
On the other hand, I believe that it is natural to search deeper connections between the three Standard Model forces, since there are some remarkable coincidences there.