While adding 2 new GCDs does help with the lackluster and boring GCD rotation, I don't think it will be enough...

Bringing back Dark Passenger as GCD combo starter that is followed up by Flood of Shadow which would then be followed up by Shadowbringer with step being Disesteem, with each step costing 1000-2000 MP and each step also heals you for 400 cure potency. We can also make Paladin's MP spending actions heal the whole party so that way they get something in terms of utility.

Moving Scarlet Delirium combo to level 62 and having Bloodspiller grant a buff that makes it so you can use Scarlet Delirium combo on execution will help Dark Knight leveling a LOT. Same with moving Impalement to level 64 and have Quietus grant a buff that lets you use Impalement. Because it takes FOREVER to get a new GCD AFTER getting Quietus.