Paladin:
Oh boy, this one used to be one of my favorites. My tank main, but then 6.3 happened, and I can't even look at it any more.
Step 1 would be to just revert everything. It was borderline perfect before, and I just want it back.
Since they're not going to do that, here's something they could do to make it less terrible. Make Goring Blade a DoT with no CD again, but have it take up all 3 Atonement stacks upon use. Atonement should also break your combo if you try to save them for just after your second combo button. Use them when you get them, or lose potency. I'm really not a fan of the 1244453 style rotation here. Speaking of, Holy Spirit should be a buff that you can stack many times. That way you can pool them for bursts, or extended movement mechanics. I'd say you should keep up to five of them optimally.
If you have Holy Spirit procs from Royal Authority, you should be able to use them under Requiescat without taking Req stacks, but still getting the damage buff. This way, it rewards you for saving them for bursts, and brings back the old feeling of using a few spells, then a few Confiteor finishers. And of course, bring back the DoT on Confiteor's finisher. obviously adjust timings to fit with this new rotation.

Black Mage:
It's in a pretty good spot as we're all aware, but I think there's a few key things to do.
Simple stuff first. give Scathe an actual use-case. as it stands, it's too much of a detriment to use because of the MP cost. We've already got plenty of movement skills if you plan them out properly. Scathe should lean into being the desperation button. Make it pause the Umbral/Astral timer for a GCD length, and cost no MP. Also remove the sharpcast trait, that was just meaningless in the first place. I don't want the skill removed entirely because I know about people running Palace of the Dead as Black Mage who make good use of it. (Masochists)
Swap the levels at which we get Leylines and Triplecast. One of these is infinitely more useful to lower level players, and I know which one I'd rather have when running old content.
This might remove too much of the fun kind of optimization difficulty, but perhaps Thunder could double-stack up to 60 seconds so we can use a Thundercloud proc for movement without fearing losing too much DoT damage.
Here's a ton of leveling changes to add onto that previous point. Fire 4 should be learned at level 50. Flare and Despair need to be swapped out entirely, or consolidated into one button. Maybe if you keep Despair, just have it on a minute CD, and increase potency so it's not the standard finisher.
(2/3)