Assuming the civil war in Skyrim has concluded, here's an outline for a possible continuation:
A moot is soon to convene. An envoy from the Elder Council has been circulating an offer amongst the Jarls, proposing to transform Skyrim into an independent kingdom under the rule of an Emperor-King. Some Jarls, having considered this offer, expelled legionnaires and stormcloaks from the forts within their holds. Should this treaty be ratified, the Skyrim legion would be disbanded and the position of the military governor abolished. The High King's role would become symbolic, with the Jarls swearing fealty directly to the Emperor-King. The "Last High King" would be elected by the moot to oversee this transition. Ulfric and/or Tullius dismiss this idea, but there is disagreement among the Jarls regarding who this Last High King should be. Thus, the seeds of political strife are sown, although, for now, without physical conflict.
Among those present at the moot is a chieftain of a High Rock Redguard tribe Silba'af. While hostilities are temporarily paused, the player visits their camp. Various quests here serve to elaborate on the logistical aspects of war. Later, this camp assumes the role of a quest hub, akin to the military camps in the civil war quest-line, especially if the player isn't aligned with the legion or the stormcloaks.
When the conflict between the legionnaires/stormcloaks and hold guards resumes, it settles into a stalemate. In this mod, soldiers require food and rest, demonstrating a sense of self-preservation and a preference for retreat over death, particularly when defending supply convoys. Casualties are relatively few, but resource attrition is significant. Essentially, the cannon fodder is just that - fodder.
At some stage, an Orc appears, seeking help with what he describes as a "vision quest." He tells a lengthy tale about how Skyrim's current predicament is due to a bodkin from Jyggalag's pauldrons, which supposedly pinned the cushion of Nirn beneath Skyrim roughly twenty-five years ago. This event is said to have 'frozen' history, particularly the hostilities in Skyrim. The player is tasked with locating and destroying the Bodkin in a dungeon. Other characters argue against its destruction, believing its preservation could facilitate technological advancement on Tamriel. The player can choose either option. Destroying the Bodkin would indeed break the stalemate, allowing the leading faction to gain momentum and ultimately triumph. However, observers would likely attribute the victory to the efforts of the winning side. Preserving it would perpetuate the challenging war, although there may be other side quests to help manage this.
Once peace is restored, the remaining factions retrospectively ponder on who really was the (Last) High King/Queen. With high enough renown this could be the Player Character. Else, who won - Ulfric, Balgruuf, Tullius, Elisif... This need not to be a moot quest, could be as simple as unlocking a new loading screen, akin to an ending result slide.
In the initial release of the mod, players will be expected to complete the civil war questline (vanilla, OCW, possibly CWORR, or use Betrayal At Hrothgar to skip it) to initiate the new narrative. An early start, branching into the new plot while both Ulfric and Tullius are still alive, is considered for future development.
While this logically expands Open Civil War to include three or more warring factions, it won't be forcefully integrated into OCW through an update. The OCW will receive its own updates, including bug fixes, interoperability and new features, along with a hint of this new story.