Legend could pull this off, so there's no reason that D&D 5e couldn't pull it off either. You should check out Legend if you haven't. They have possibly the most innovative multi classing system I have seen anywhere. Each class in Legend receives three "tracks" these "tracks" are buckets of abilities that you receive over the course of leveling up. Each level (other than first) gives you an ability from one track. Each track has a theme that unites the track. Paladins have smiting and ally buffing tracks. Barbarians have rage and toughness increasing tracks. Multi classing works by swapping one of your tracks for another track from a different class or from additional tracks not attached to any class. Some tracks are passive abilities, while others are active abilities. So you give the "I want a simple class" guy a class filled with passive tracks boosting attacks, defenses, movement modes, whatever is appropriate for the class in question. Then you let the "more complexity" guy choose from more active tracks. Of course this could all fall down if "I want a simple class" guy decides that a class ability every level is too many even if they are passive abilities like "do more damage", "take less damage", "boost your defenses", "move faster". I'm still optimistic about it though. It has been done before. WotC could pull this off. Or we could all just take up playing Legend instead.
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Art from GIS for Life of Brian