I'm sure I'm not the first to come up with the idea and I probably won't be the last.
Giving Paladins a Block Value to spell damage conversion would grant several positive benefits at once.
First of all, Paladins would have a synergistic stat for Threat and Mitigation. Currently, Paladins are the only tank that does not have a stat that gives both Threat and Mitigation. It would also have the side benefit of being the exact same synergistic stat as the other plate wearing tank, the Warrior. So now both plate wearing tank classes would receive mitigation and threat from Block Value.
This would allow the developers to further their goal of having one set of gear that appealed to many different specs. You would basically need 1 set of Plate Tank gear that would be used by both Paladin and Warrior tanks.
Any flavor difference could be addressed in Tier gear, such as the Paladin's need for a very small amount of Intellect or Spell hit.
Furthermore, it would have the side effect of allowing Paladins to use the existing Tanking weapons already in game. There would be no need for a specific Paladin tank weapon as some have asked for. Paladin's use their weapon slot to gain the lion share of spell damage they have on their gear. With that spell damage now coming from Block Value, they wouldn't need to use Caster weapons.
It would allow the Paladin's threat to scale better than it does now.
Another consideration, is that it would allow for a much smoother transition through the leveling curve and into the end game. Currently, spell damage plate doesn't really exist in the game until you reach Outland, making it a difficult transition for Paladin tanks. They have to tank one way leveling up then completely change their approach once they reach Outland. With the Block Value to Spell Damage conversion, they would simply use the Warrior gear all the way up and continue gearing that way in the end game. The spell damage they need would come naturally from their gear.
I haven't seen any potential negative side effects of making this change, so I hope the developers will consider it for either a future patch or perhaps for the Expansion.