Isn’t one of the core values of being a Paladin to help or heal your allies? I think ret pals Mastery should reflect that with a small raid wide healing based on your damage done; similar to Vampiric Embrace circa WotLK.
Arcane Mage is probably my favorite Mastery right now out of all the specs. It adds spec flavor by increasing your resource pool and regen rate while also increasing the damage (albeit not a lot) of your affected spells because it’s a damage spec.
Right now a lot of classes mastery’s are pretty boring. A flat % increase to holy damage done is really vanilla and doesn’t really add much to the gameplay. If anything, Mastery should encompass that spec’s flavor and reward you for putting more points into the stat. With Shadowlands around the corner I think it would be a great opportunity to take a look at class Masteries and reevaluate how they reflect on the class/spec fantasy.