In a "grand unified theory" (which does not include gravity) the strong, electromagnetic and weak forces are unified into one gague theory. SU(5) is one choice. In these theories, the electron, quark and neurtinos fit together as if they were different versions of the same particle, just as in the standard model the up quark has three "colors". In these theories there is a well defined relation between the charges. You can lookup the SU(5) unified theory to see more. I would say these theories are widely believed, but we have not managed to put them all together yet.