Orthodoxy did not exist as a totalitarian entity, although each type of Christianity may have thought of itself as orthodox while everyone else were heretics. So the discussion of heresiology is important to maintain, as long as one understands that the heretic is so only from the point of view of one party. An orthodox Christianity doesn't emerge until the fourth century. Even then, it struggles through council after council, swinging from Arian to anti-Arian for over fifty years. Not until the fifth century are the major lines put into place that will determine the shape of "orthodox" Christianity for the centuries to come.
Heterodoxy is not any better because it describes religions that deviate from the orthodox. Since we don't have orthodoxy yet, we can't have heterodoxy either.
Sectarian and cult language don't work either, because sectarian requires that there is some parental tradition that is being deviated from. Cult also suggests deviance along with innovation.
So what do we have? Multiple forms of Christianity, although this isn't quite right either, because many of these forms are competing with each other and some forms of Christianity are stronger and more dominant in certain geographical locales. So what we have is plurodoxy. That is multiple forms of Christianity that are competing for the orthodox position and/or that consider themselves to be the orthodox position. From this vantage point I think we can better narrative Christian origins and the standardization of Christianity that eventually comes to dominate as orthodoxy in the fourth and fifth centuries.