My idea (or at least my hope) is that Clerics with the Protection Domain receive:
1. The Protection Fighting Style ability to impose disadvantage on enemy attack rolls per turn as their initial Domain ability.
2. The same (or similar) Channel Divinity power in previous playtests where you negate damage to an ally at range.
3. Heavy armor and shield proficiency, but only simple weapons.
I want to be an immovable wall between the enemy and my allies, and I don't mind sacrificing killing power for it. I always liked the image of a heavy armored mace-and-shield wielding holy warrior.