I reakon Coruscant is a good bet. When we see Dooku vist Sidius it was in the same firey/industrial part of the city where Anakin and Obi flew over the lava.
They've already said that the fight takes place through a number of sets/rooms i.e. not just on a vulcano but within a building that has lava in/around/beneath it.
So coruscant is my hot favourate, but with the promise of so many new worlds it could be anywhere. Lets just hope its not in a lava pool where the workers are trying to get the Calibar crystal under Anakin's command, and Obi stumbles across it and tries to stop them. No crystals! No crystals! No crystals! Come on, chant it with me, No crystals!