When it comes to marketing a Black Marsh game, I think they should emphasis the danger and strangeness of Argonia and the Saxhleel. Look at Argonia from a Cyrodiil's perspective, then travel the land and get a good sense of its culture, its people, its dangers. Trailers showcasing battles between Imperials and Argonian auxiliaries against other Argonians, a deadly armored Wamasu sealing the fate of the Imperial forces. Or a Dunmer protagonist for a trailer, facing off against a pack of deadly creatures in Arnesia. Or, to borrow from Beyond Skyrim's lore book "Heartwood of Darkness", a group of Dunmer fighting Argonians in some ruined city (Tear), only to fight their way to the heart of the city and come upon a Hist, a horrifying sight, creating abominations of the Saxhleel form, and perhaps some Argonian-like creature, looking nothing short of a Weredaedroth, rising up from the murky waters of the sunken district to meet them.
The idea I had envisioned in the original setting-concept piece was an inter-tribal war between the Saxhleel tribes, fought mainly by two factions. the An-Xileel - fierce traditionalists who's priest-kings interpret the will of the Hist to guide the nation, and the Archeins - a once powerful tribe that infamously prospered through collaboration with the Cyrodiils and the Dunmer. Seeing the threat posed by the rogue Hist - Hist that have for some reason become corrupted/removed from the Hive-Mind, launch an Expeditionary Force to Black Marsh to restore order, deal with the rogue Hist and install the Archeins as the rulers of Argonia. As such, the Nibenese settlers, along with the Expeditionary Force, along of course Dry-Skin mercenaries and merchants, would make up the bulk of the humans in the game. The Dunmer would have their own story dealing with the north and a guerrilla war to retake Tear by Great House Dres. Perhaps include Great House Sadras as well, a Dunmeri alternative to supporting the Dres, perhaps some sort of pro-Imperial/pro-Peace purpose, anything that would be good for business, they are the heirs of House Hlaalu's seat after all.
I also have this idea kicking around that the Expeditionary Force, due to a lack of men the Elder Council can divert from the border with the Dominion, is made up mainly by conscripted prisoners and Argonian and Dunmer auxiliaries. As for who leads them, I was thinking that the Elder Council drudged up a relic from the past, an order of knights disliked by the City for their zealotry, especially in the wake of the signing of the White-Gold Concordat. Seeing the signing as the Mede's betrayal of Talos, they may conspire with members of the Elder Council against this heretic dynasty. Yes, I speak of the Red Dome Templars. Perhaps a sort of "Talos cult" among the Red Templar leaders.