To continue the Black Marsh discussion, aside from the Argonians and their various variations, such as the assimilated slavers and advisers of the Archiens, the toad-like Paatru, the "winged" Sarpa, the needle-mouthed Agacephs and the snake-like Naga, there are the Hist, which would most likely play a large part in the game. Black Marsh has also been host to many other races, form the possibly extinct vulpine beast folk known as the Lilmothiit, though, they could have endured in some form, perhaps as a tribal culture, living on ships and floating islands.
Northern Black Marsh could also be home to many Dunmer, as the Argonians did reclaim some ancestral land during the Accession War, perhaps even the now ruined city of Tear, perhaps bringing in the decimated House Dres and Velothi tribes into the game. It would be fun. And while the Empire doesn't seem to have much presence in Black Marsh at the moment, while it was part of the Empire, they certainly left their mark. While mostly flooded, cities like Gideon, Lilmoth and possibly Soultrest included Imperial architecture and influences. With Skyrim a liability and the Hist posing a significant threat, they could have sent an Expeditionary Force to Black Marsh, perhaps aided by what few allies they have in Morrowind, to neutralize the threat and perhaps even bring the Argonains, or at least the Archiens into the fold. The Thalmor's interests in Black Marsh could have also caught their attention.
As for geography, which a significant portion of Black Marsh is marshlands, it is far from just an ordinary swamp. There are some mountains, and Black Marsh's north-west is where Nibenay and Deshaan intercede, so we might see aspects of Morrowind in Black Marsh. No snowy mountains however. As for legendary creatures that would be a must, the Swamp Leviathan, the Wamasu and the Rootworms. Seeing Sload would be fun as well, as they supposedly operate in coastal regions. We could also see Maormer stalking the southern shores, and perhaps the Sky Renders from the Redguard comic.
A question, for a Black Marsh game, would people want Tear and part of Deshaan included in the game? Since it is probably going to have reclaimed Arnesia, they might as well throw in the ruins of Tear and have a story involving House Dres. Could even include Narsis and give the Dunmer a larger presence, to better portray the Dunmer-Argonian conflict.