ChaosOS
Legend
Here's another one: Weapon proficiencies. I can't think of anything that would be broken by allowing all classes to have proficiency with simple and martial weapons. Weapon-using clerics would get slightly better, but they needed the buff anyway.
Re: The dominant argument. I'm inclined to agree with the position that 5e practically doesn't have class skill proficiencies, and the existing ones mostly just guide players into the class archetypes.
Now, for weapon proficiencies, as someone who's played a weapon cleric I disagree that they "need a buff". Cleric is solidly one of the strongest if not the strongest classes in 5e, and with Spiritual Weapon can keep up with most melee characters for DPR if the days stay on the shorter side (which I find is much, much more common than days that tend long and would drain spell slots). Combine that with the cleric's utility belt massive prepared list that's not even restricted by a spellbook, and you've got a class that already threatens real melee class niches.