On his Facebook page, Soule responded to a fan's query about his future projects: "No, I won't be returning to Guild Wars. EverQuest has an exclusive with me for the future projects that involve MMOs. [Guild Wars] was a good experience over many years. As with all good things, it had to come to an end."
Soule was the main composer for Guild Wars and Guild Wars 2. He has also contributed to The Elder Scrolls Online main theme but will not be doing any other music for that title.
[Thanks to Bogotter for the tip!]
[Update 3/12/2014: Soule's agent has contacted us to clarify that his exclusivity to the EverQuest games is lime-limited and is unrelated to his abbreviated work on ESO. We may yet see him return to the Elder Scrolls franchise in the future!]