Basically, think of a movie that works along the same lines as the animated TV series, Tron Uprising, which saw new characters affected by Clu’s evil regime in a time period before Tron Legacy.
With some fresh narrative data circulating through the Tron universe, a successful side story would be perfect justification to revive the dormant sequel somewhere in the future, bringing everyone back to the Grid proper.
Since Ares is listed as a “key player” in Tron Ascension’s script, this new story could develop the character for future films, while weaving in some of the threads that Tron Legacy didn’t use to their fullest potential.