By the OP's description, I'd say 4. I'd love to see the new dungeons be a dramatic step up in difficulty and iLvl, where running the dungeons is a pre-req to get into Final Coil. I'd like to see the new dungeons drop i115 loot, with an i100 or i105 requirement to enter. Final Coil, meanwhile, would be i110 requirement to enter and dropping i130 stuff.

The new tomes in the new dungeons would be spent on new i115 accessories as a counterpart to the i115 armor drops.