[Autumn]

I believe the most satisfying conditions for monoclassing are:

-Seriously strong passives that boost the power of the class to an all time high, nearly doubling their effectiveness
-Being able to compete on the levels of dual classed characters but not necessarily overshadowing them completely.

This can be achieved by improving currently existing mono-class benefits and even introducing some for say the likes of Kensei or Demon Hunter or Black Knight

Or by introducing specifically mono-classed skills that have egregious power budgets, given they're restricted to solely being mono-classed.