Each system, particularly D&D 5E has optional rules. The GM should be able to select which optional rules are used, and when they replace other rules, the other rules should be removed or overridden. For example, Tasha's Cauldron of Everything established numerous optional rules for all the character classes. The GM should be able to select which optional rules apply in their game just like the GM can select which pieces of the Compendium to allow. This would allow the GM to select, for instance, the optional rules for Rangers (which would completely overwrite the regular rules, where appropriate) but not the optional rules for other characters. Or vice versa.