This forum is a fan theory, based on the similarities uniquely shared between these two franchises. This can be possible under the speculation that, as there are no maps showing the entire lamdmass of which Nosgoth (LoK) is part of, and only a few with a single coast line to the south and unexplored land outside the country's boarders, Nosgoth could be a small, southern region of the mysterious and unexplored Akavir. And that the LoK timeline takes place before TES.
Hope you all enjoy!

Pillar Guardians/Divines
This one might be a lil farfetched, but still plausible if you have any understanding with the history of world religions. The Creation of Mundus, the Aedra and the Daedra could be a fabrication of the war between the Vampyres and the Hylden. The ancient vampyres in LoK were once angelic beings, until their religious bloodfeud drove them to banish the Hylden to the Demon Realm. The Hylden retaliated by afflicting them with a blood curse, which turned the ancients hemovorous, immortal and sterile. This drove all but one (Janos Audron; see Heart of Darkness/Heart of Lorkhan below) to commit suicide. After banishing their adversaries, the Vampyres erected 9 pillars to bind their banishment, and assigned 1 guardian to serve each pillar. Which they later passed on to an apprentice. This could be romanticized over time as the Aedra (Vampyres) sacrificing themselves, and the Daedra (Hylden) escaping to Oblivion. As there were originally 8 Divines before incorporating Talos, so too were only 8 guardians ever shown in LoK. The 9th original Balance Guardian, whom stood at the center of the rest, never made an appearance. The position of balance guardian has changed more frequently than the others, leaving this as a possible niche where Talos filled in as the 9th divine. My best guess as to match Divines to pillars:
Akatosh = Time
Dibella = Energy
Mara = States
Julianos = Mind
Kynareth = Nature
Zenithar = Dimension
Stendarr = Conflict
Arkay = Death
Talos = Balance

Heart of Darkness/Heart of Lorkhan
Another possibly farfetched comparison, merely interpreted by a relic of great power, said to be the disembodied heart from a long deceased divine being. The Heart of Darkness once belonging to the last of the Ancient Vampyres, Janos Audron, before it was ripped out of him and continued beating. It was then taken, hidden and used with the belief it could restore/bestow vampiric unlife.

The Elder God/Hermaeus Mora
Their identical design says it all; mass of tentacles and eyes. Even the eyes themselves match up perfectly with their hourglass shaped pupils. Either they are two separate but related entities, or they are the same being. In the latter, though their goals seem to differ, they could been seen as the same. The Elder God being the self-proclaimed embodiment of the "Wheel of Death and Rebirth," fed on souls to cleanse and process them over and over again. This could merely be the method to meet Hermaeus Mora's goal to obtain knowledge; filtering souls to obtain their life's knowledge and secrets, wiping their memories to be reborn, and repeat.

Meridian, and the Seer/Meridia
Self-explanatory. The capital of Nosgoth could possibly be named after the Daedric Prince herself. Though, I mentioned the daedric princes possibly being Hylden, her angelic appearance in certain statues/artwork is shared with the ancient vampyres. Could just be a guise. Could be she was a banished vampyr. Could be a connection with the Seer (Blood Omen 2), whose backstory we never got the chance to explore and whose fate remains undetermined; she was presumably a vampiric Hylden, with notable fangs and appeared as the Hylden did pre-banishment.

Spectral & Demon Realms/Oblivion Realms
While TES has numerous interdimensional planes of existence, ie. "Pockets of Oblivion," LoK likewise had similar realms. "The Demon Dimension" where the Hylden were banished, and "the Spirit/Spectral Realm," which could possibly be the equivalent to the Soul Cairn.

Hylden & Mortanius/Necromancers Bone Goliath
After the release of the necromancer class with the Elsweyr dlc, I noticed a resemblance between the design of the Bone Goliath ultimate and the Hylden from LoK: Blood Omen 2. Furthermore, the only necromancer to appear in the franchise, Mortanius was possessed by a Hylden and was forced to transform into a hulking demonic entity by the end of the first game, Blood Omen; being a very old game in the series, the design was later given to a separate race of demons and the design of the Hylden changed and were not seen again until Blood Omen 2 (3 games later), but the entity possessing Mortanius remained of Hylden origin. Not to mention those possessed by or corrupted by the Hylden had a similar greenish aura or glowing from their eyes as the greenish/bluish coloration seen necromancer skills. Furthermore, though the Demon Realm the Hylden were banished to (as seen in LoK: Defiance) looks closer to the Deadlands than Coldharbour, it is possible that there is a connection with Molag Bal, due to his followers are often Necromancers and his features shared with the Hylden/Goliaths.

Razielim/Vampyr Lords
The Razielim were the only clan of vampyres missing in action in the Legacy of Kain; unless anyone would count the cancelled MMO Nosgoth, by which the playable vampyres are supposedly "variants" of their respective clans. One theory suggests that as they were being hunted down by the other clans, following Raziel's execution, the remaining Razielim escaped and set sail to Tamriel. Soon afterwards, Molag Bal took an interest in these refugees and collected the remaining members to perform his experiments on them and corrupt their vampiric strain for his own means. The first victim to be afflicted with his new-found vampirism being Lamae Bal. While, others who sought out this "gift" became Vampyr Lords and were granted the winged adaptation of the original clan members and their founder, Raziel, Lamae developed the wingless Scions, to mock Molag Bal's winged creations. Nevertheless, the wings would be lost as the bloodline thinned. It's worth noting that Molag Bal's version of vampirism is immune to water, which burns like acid to vampyres in LoK. However, in hindsight to Skyrim: Dawnguard, when in Vampyr Lord form, the player never submerged or even touched the water. They would either hover or walk on water, with a faint aura beneath their feet, as if to avoid it's touch. While not in Vampyr form, one could argue only that the weakness to water might only be prominent when in their true forms, similar to how werewolf players in ESO are only vulnerable to poison damage when in werewolf form.

Kain/Vampyr Lords
An alternative theory, following TES lore that Molag Bal did, in fact, create the first vampyres of Tamriel. Doing so by way of necromancy, likened to Kain's creation as a vampyr, though, without the Heart of Darkness. Molag Bal could use the same or similar blood curse the Hylden afflicted the ancient vampyres with. Looking at the similar features in ESO's Vampyr Lords, Blood Scions and BloodKnights to the elder Kain's, it is possible that Kain might continue to evolve to look more like them. We never see how any other vampyres are created, only hunted at by Vorador that "it takes time and energy to create a vampyr". Presumably, Kain's creation was very unique, as is his appearance, and though he used a similar method to "raise" his lieutenants, he only bestowed each of them with a unique "gift". These gifts would be reflected in their monstrous evolutions, and passed down to their clans. As Raziel developed wings, this may be a from Kain's bat-swarm ability, or it is possible Kain will likewise develop wings as he continues to evolve. Plus, Lady Thorn's monster helm, equipped with a dark elf or ashlander style sword (see below), will give the player a near Kain-like appearance. In addition, without the Ancients' essence mixed in with the Tamrielic vampyres, we wouldn't see the most common feature all vampyres eventually develop over time in LoK: cloven hands and feet. As well as vulnerability to water, as is it unknown if this was a side effect from the curse or a previous weakness of the Ancients.

Rahabim/Tsaesci
Returning to the possibility of counting the cancelled MMO Nosgoth, Rahabim were the only ones unseen as a playable race. It's possible the Tsaesci ("vampiric snake men") could have been the missing variants to the Rahabim. This could be support by the design of the Rahabim in Soul Reaver. They were an amphibious, reptilian species of vampyr, with fins going from their brows and down their backs, in a cobra-like hood design. They, with human thralls, could have sailed across the sea to Tamriel, and while the humans eventually mingled into Tamrielic bloodlines, the Tsaesci either left or dwindled over time.

Wraiths
Though ghostly cloaked figures hovering off the ground are hardly unique to either franchise, small details between them landed them in this list. Both feed on one's vitality (except the necromancer skill pet which restores vitality), and if you look under their hoods, they both have glowing eyes and fangs. Though, Raziel's appearance in LoK differed from the others, the other "wraiths" in LoK: SoulReaver were the souls of dispatched vampyres. However, it is never specified in the series if only vampyres could become wraiths after death.

Shades
With the release of the Dark Heart of Skyrim, we were introduced to a new kind of spirit; shades. Shades, in ESO, are spirits trapped in the void. Like the wraiths, they share some similarities to their LoK counterparts. In LoK:Soul Reaver2, we first meet the shades as dark, semitranslucent humanoids with glowing, heart-like centers. They underwent a dramatic redesign in LoK:Defiance, though, as dark clouds with hands, and one to three glowing "eyes". Whether this was simply a redesign or possibly a shade variant, the Void Shades bear a closer resemblance to the former; dark, robe-like bodies with a vibrant glow in the opening to the face and chest.

The Soul Reaver/Dark Elf word style
The design of the Dark Elf and Ashlander sword styles are nearly identical to that of the SoulReaver, minus its signature skull chappe.