I think that with the Obi-Wan VS. Vader pieces in the PF line, and the ROTS Anakin/Vader VS. Obi-Wan 12" figures, they have shown that they know how we want to display these things--recreating famous duels. The fact that Qui-Gon and Darth Maul have been produced, and the fact that Maul includes his severed saber, makes me think that we won't have long to wait until the Duel of the Fates is complete, giving us three of the most memorable lightsaber duels from the saga in Sideshow form.
I would say that 12" Luke VS. ROTJ Vader will be the fourth duel to be realized, followed by Mace VS. Palpatine, and then Anakin & Obi-Wan VS. Dooku from ROTS. After that, Yoda VS. Palpatine from ROTS and Yoda VS. Dooku in AOTC--once they figure out how to do Yoda properly.