Skyrim is undeniably one of the most popular games to ever grace the market, and all the different editions and ports it got over time show just as much. The game was expanded with DLCs, adding to its hundreds of hours of content as is, and then the dedication of Skyrim's modding community doubled down on that even more. With this in mind, however, it's quite easy to see how The Elder Scrolls 6 will have a hard time living up and surpassing Skyrim because the latter's legacy is one that will turn ten years old, and still thriving, this November.
Very little is known at this point about The Elder Scrolls 6, which was announced over 1110 days ago at E3 with a short teaser trailer, and then was kept on the back burner by Bethesda due to Starfield being the studio's priority. After three years of waiting, fans of the Elder Scrolls saga have yet to learn what the setting for the next installment of the series is going to be, but Starfield seemingly recently teased it being High Rock and Hammerfell. Whether this will be the case is still up in the air, but if that proves to be correct, then it's safe to assume that some enemies in Skyrim aren't likely to show up again in The Elder Scrolls 6.
Draugr
The Draugr are among Skyrim's most iconic enemy types, and they are undead Nordic warriors of the region that mostly appear in catacombs or similar dungeons scattered throughout the game. Draugr are very scary warriors in terms of their strengths, and while most basic Draugr will simply wield Ancient Nord weapons and attack the player, some more powerful undead also have the ability to use Dragon Shouts or conjuring spells to have Frost Atronachs help them in the fight. Even death doesn't seem to take from the Draugr social hierarchies, and in fact, some of these undead are addressed as Deathlords or Death Overlords.
This enemy type is, unfortunately, bound to the region of Skyrim and to Nordic crypts, which makes it quite unlikely that they will return in The Elder Scrolls 6. However, with both High Rock and Hammerfell being the rumored setting for the next Elder Scrolls game, it should be noted that both regions are adjacent to Skyrim, which could allow Bethesda to make Draugr have brief appearances. It's unlikely, though, as it wouldn't really fit in terms of worldbuilding.
Falmer
The Falmer are a very interesting race of Mer, known as Snow Elves prior to the population's regression. Falmer are blind, but they have incredible hearing and smell that can quickly alert the creatures about danger or prey. Much like the Draugr, the Falmer also retained ranks and roles, despite the fact that they previously were a more advanced and diverse people that likely resembled the current Mer races.
Because of climate reasons, these creatures are also unlikely to come back to The Elder Scrolls 6. If TES 6 is set in High Rock and Hammerfell, there's not much "Snow" there to correlate the race with. It's not impossible, as Giants appear in both Skyrim and High Rock, but it wouldn't really mesh.
Dragon Priests
These powerful undead enemies are the least likely to return in The Elder Scrolls 6, and that's because of their innate connection to the lands of Skyrim. These priests used to be the most feared rulers of Skyrim apart from their Dragon masters, and it is possible that the warriors they commanded were the Draugr. These enemies are a crucial part of Skyrim's quests and there are 8 of them in the main game, all laying in wait for Alduin, the dragon called "World-Eater," to awake once again.
Because of these reasons, while the Dragon Priests are very compelling villains to fight, it is also implausible that they could return in the next installment of the series. Regardless, it is a safe bet that TES 6 will be a huge game with lots to offer, and while it should build on some of the extremely valuable foundations of its predecessor, it should try to be its own game rather than the sequel to Skyrim.
The Elder Scrolls 6 is in development.