Most of these are directly lifted from some old, unrelated worldbuilding I did, but I think they introduce interesting possibilities to the Black Marsh, which will be relevant as the first dawns cast light on the southernmost reaches of TR. They all create a central theme of the Hist struggling to control a Black Marsh that has many, many independent processes threatening collapse of the whole marsh.
Although this would mostly apply to the Black Marsh itself, I could see this having use in some of the furthest southern reaches of TR. The concept is that the Black Marsh has a type of tree whose roots are many, strong, and which consume a lot of water. These roots essentially create little patches of solid land (whether it's normal dirt or all treeroots, idc, but treeroots are better flavor imo) dotted around the Black Marsh. Given dunmeri lifestyles and sensibilities, Dres slave traders would value these patches enormously as a preciously scarce platform to build camps on during slave raids. And, especially given that these jungles are mostly ... out of view ... of law enforcement, the feuds and conflict over these patches would be red hot, providing TR a good avenue for more slaver characterization, and likely many novel quests that send the player off into the marshes. (the few marshes that will exist in TR, of course. At least for the foreseeable future.)
An idea that doesn't fit as conservatively in TES was to have a huge worm under the ground that creates and consumes alligator-like worms that leech all the nutrients from the top of the swamp, and cause a swathe of marsh to be a ghostly, lifeless white. Unfortunately, they spread, so the Argonians and the Hist have the nightmare of digging 60 meters or so down to the worm (in marsh mud) (with the alligator worms swimming around in the dirt like sharks, and they get more numerous when the mega-worm notices that something's up). These cleanings routinely cost hundreds, or even thousands, of Argonians, but any one of them could spell the death of the whole Black Marsh if it got out of control, which forces the many factions of the Hist to work together, and at least in that capacity, act as one entity.
Here's where the concept porting to TES doesn't work as well anymore, but is still, maybe, workable.
Helicans, Helican Monsters: (capitalization is wrong/weird for now)
This is a concept even further from TES, and impossible to do full justice to in practice. A "Helican" was a small roughly triangular nodule of cartilege surrounding a core (originally metallic, but magic didn't exist in the original worldbuilding, so we can discard that workaround and make it fleshy if we want). Each Helican doesn't accomplish much by itself, but Helicans aren't independent creatures, as much as they compose independent creatures. With each helican, the helican monster gets stronger, and more importantly, smarter. If they got out of control, they would compose gigantic beasts with intelligence entire categories beyond the highest brilliances of man and mer. And, because their forms are all floating helicans, they have greater dexterity than hominids, and have the brainpower to use this dexterity to its fullest, doing many complex things concurrently. I've never figured out a reproduction strategy for helicans, so I'm thinking that they can't be created or destroyed, and their communion is a possibility terrifying to the Hist, but the Helican Monsters' absurdly high competence makes them an appealing target to naive argonians/dunmer to enslave. (Imperials aren't aware of this) One "kills" a Helican Monster only by seperating its helicans until they no longer compose a monster, but doing so is extremely tricky and dangerous, though Telvanni mages could be an excellent tool of last resort; they can levitate (the helicans float, so they could concievably fly), and have telekinesis. To take down a mature monster would take much more than one mage though. Fortunately, the Hist have done a good job, and the threat of a Helican catastrophy is a remote, abstract possibility. Still, it's a world-ender threat, and greedy, naive characters do exist. There may be room to develop the Hist deal with this idea.
Although this idea has to be played down a bit, (the original were artificially created creatures who gave birth to a variety of organic guns, depending on what (artificial) sperm was used (my apologies if that's in bad taste here). The guns most commonly produced were beautiful alabaster guns that fired a few shots, and made for a heavy club. The club wasn't for after running out of ammunition though; rather it was to conserve ammunition, because ammunition was more scarce than guns. (Bows hadn't been invented yet, so this impracticality was accepted) All of this because the intelligence distribution was not a bell, and there was a distinct mutation that caused "wizards" who were stupidly smart, and invented things independently of everyone else's tech.) A creature like this could be created by the Hist, and it produces some exotic Argonian weapon not seen anywhere else. (Maybe we could use the Atlatl to add a novel ranged weapon to the mix.)
This doesn't fit as well with the marsh outside of the motif in these ideas of things exponentially slipping out of control, but it suits that pretty well, and helps to validate the austerity of the Black Marsh. Basically, in the original version, people bred dogs to do all of their war for them, but the people using the dogs became too weak-willed to control their own dogs, and they broke into the environment around them, becoming man-eating, aggressive animals familiar with human behavior, who aren't supported well by the local ecosystem, and are accordingly desperate to eat. In the original idea, the people never left their towns, because it was certain death-by-dog to do so, and over time, they came to live mostly underground. I'm not sure how this can be translated to TES, but it fits well with the motif of losing control to the wilds of the marsh. Perhaps the Hist tried to perfect the art of the warrior argonian, and accidentally created this monster in the process, but the warrior's incorruptibility made its mind inaccessible to the Hist, and the strategy for recovering losses was reproduction, or something like that. The function of these monsters, imo, depends on how we choose to characterize the Hist.
All of this flavors the Black Marsh a lot, and gives the Hist a new dimension as stewards of an unruly, uncontrollable marsh. How well they work together can also be put on display here, and could inform the Dres' relationship with the Hist and with Argonians more generally.