D&D 5E - Proficiency bonus based AC and armor as temp HP with reduced AC bonus?

Something with which I've toyed is allowing Fighters to use their Proficiency Bonus instead of Dex for AC, and possibly for Initiative later. This is in line with 'Hollywood Heroics' where the protagonists are usually lightly armoured. A PC can still have a max AC of 21 (Studded leather, shield, +6 PB, +1 Defensive style) without magic. But PCs can still don plate armour if necessary (for instance on a large battlefield, a PC might be denied her Dex / PB bonus and thus lose a lot of AC otherwise). I would extend this to allowing Palading to use similarly their Cha bonus instead of their Dex bonus.

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