Physicists can write such theories today, but more data is needed to tell which of the many versions, if any, decribes nature.
Such a theory would tell us at what energy scale all the forces merge into one.
It predicts new force-carrier particles that could cause the proton to decay. Such decays must be extremely rare; otherwise our world would not not exist today. Measurement tells us the lifetime of the proton is greater than (10 to the power 32) years!
