I don't think paladin is that bad at mitigating damage, at least arguably better then warrior at raw mitigation both have similar less mitigations in trade for self sustain, I feel the biggest problem with Paladin is that a lot of its self healing isn't up to the Paladin it's just on Req, which is pretty bad for raids, not saying the req healing you get isn't powerful but having no control of it means it gets wasted in a lot of situations.
Shelltron Should become a 20% damage reduction instead of a block could also target a party member, Oath shouldn't be based on auto attacks, a Timer around 45 seconds to fill to 100, while in combat would be much better for PLD to regen its mitigations during down time). I wouldn't mind seeing Paladin given another personal but generally I would rather A lot of req's sustain be moved onto a stronger regen effect on holy shelltron so that Paladins have more control when they regen.
Things Like Clemency should generally stay as it is, it being a GCD spell gives Paladin a choice in situations where healers are struggling or you got none, It gives Paladin the Opportunity to work as a backup healer, Instead you could argue that giving Paladin a separate Heal as a OGCD could help self sustain and party sustain, Gunbreaker gets a regen I don't see why it would really break the game if paladin had something similar outside its short cd and self healing (if plds holy spirit/blades phase didn't heal as much) I dont really see the problem with it, Things such as Shield Bash and Intervention, could be removed to get rid of some bloat on paladin to make up for a extra ability (IF holy shelltron was targetable)
We should keep in mind that implementing too much would overtune Paladin defensively, that could be balanced by having PLD have slightly less dps (Not by a lot, Paladin currently doesn't do enough by large amount). I want the Job to feel like the Holy Knight, with white magic abilities. Obviously we should be looking to balance any of these suggested changes I really don't want a situation where Paladin outclasses all the other tanks defensively.