TLR - in current state of wow undead paladins are unlikely. With a tiny bit of lore twisting/creating - they would make more sense than sunwalkers.
I love the "Dark Paladin" or "Shadow Paladin" idea. Saying the UD Pally uses Shadow magic the same way the UD Priest does to heal. Lore would be explained that they are not using the Light, and Gameplay mechanics would stay intact with the understanding that melee and spells are shadow, not Light.
“Once you have tasted flight, you will forever walk the earth with your eyes turned skyward,
for there you have been, and there you will always long to return.” Leonardo da Vinci