PIE could be interesting, but it's uses will be limited to melded gear regardless. I can't see them adding PIE to any normal tank gear, even DRK's AF. There was interesting theory crafting going on at one point early in 2.4 that BLM could get another Fire 1 with enough PIE every rotation, and I think we might see something similar to that with DRK if it uses boons of MP for it's spells. Unlike PIE, I don't think Spell Speed will have the same effect, unless SpS changes to regenerate more MP or something.

Also, I'm glad to hear that the speculation I posted was in the right ballpark. I especially like Myth's idea of sacrificing health to regain MP, to then use more spells - that sounds like a really fun system to mess with and would add some much needed variety/risk to tanking. I do hope they incorporate blood in some way and don't just drop it from DRK completely.