Todd Roybark
Hero
I wonder now if the answer hasn't been looking at us since the beginning of the game: different XP tables for different classes. Require more XP to level up more powerful classes instead of trying to make all characters fit a power level
This is the answer only to the Jeopardy question of ” What prior D&D rules do you not want to see implemented again”
It also would not help that much. Take the Arcane Trickster subclass for the Rogue. Upon initial release many considered the subclass to be weak. Add in Booming Blade, the ‘Killer App’ of 5e, and sprinkle in a pinch of Warcaster Feat and Elven Accuracy Feat, and suddenly the Invisible Arcane Trickster is the 5e equivalent of a tactical nuke in Minesweeper.
Also 2e Bards were often better casters than M/Us due to being 1-2 levels higher than a M/U because of the decreased XP requirements to level.