So one thing I suggested in one thread a while ago, was to give D&D something like the journey mechanic from Adventures in Middle-Earth. And if such a system existed, then I feel some classes, ranger in particular, should have features that interact with this system.
Similarly, if a complicated social combat mechanics existed, then I feel some classes, bard in particular, should have features that interact with it.
And once such integration to the classes is done, you no longer can easily just decide to not use the system, as you deprive classes of their features. Like if you decided not to use the combat subsystem a lot of features would become useless. Highly modular game would best work as a classless game, as then you could freely build the characters only with the building blocks that interact with the mechanics you intend to use. But that would cease to be recognisable D&D, and would greatly affect the ease of use.
Game design is a bit harder than a lot people here seem to think.