Just remove feats and multiclassing. They are optional and they are pointless. 5e characters are already strong without the need to optimize AND archetypes/races already give you plently of variety.
Problem solved. Moving on.
Multiclass by itself in 5e has a few powerfull build but the most powerful ones are straight class with feats in fact for melee Mainly Fighters and Barbarians can be a handful but Fighters are the worse