Like many of you, I was a fan of the main Elder Scrolls games long before I ever played ESO. Like many of you, I believe that the "RPG" in "MMORPG" (especially in a TES game) is still important to the genre. And, like many of you, I find that there are precious few opportunities for roleplaying actions to make a meaningful difference to end-game gameplay.
As such, I'd like to make a modest suggestion for expanding the system of quest-linked Miscellaneous Buffs, found below:

As you can see, both the number of buffs and their general utility is... rather limited. At present, they are a vestigial system giving a small nod to the legacy of roleplaying in the Elder Scrolls series without making a meaningful mark on gameplay. Some might say that it is best kept that way, but I disagree.
It is true that handing out a large number of permanent, stacking buffs from quests might not be advisable, but it does not need to be handled that way. In fact, we see this in the following quest reward for "Bosmer Insight" (a quest about being initiated into a Bosmer tribe and choosing your Spirit Animal) in Grahtwood:

The idea introduced here is to have buffs be
mutually exclusive and for each prospective buff to best fit with one of the defined Elder Scrolls class/role archetypes so that different characters have different reasons to select different buffs.
The above might not be the best embodiment of that. The quest explains the three potential Spirit Animals: "The Snake uses stealth, the Tiger uses strength and the Wolf uses teamwork." So instead, we might have buffs like:
- Snake's Scales: Decreased detection radius while in Stealth by 1 meter
- Tiger's Fur: Decreases the cost of Block and increases the amount of damage that you can block by 3%
- Wolf's Pelt: Increases the duration of all Major and Minor buffs that you apply to allies by 10%
The idea is to give certain roles and playstyles a small but noticeable boost while being mindful to avoid obvious sources of power-creep (such as raw damage modifiers) or buffs that are too universally desirable. The buffs should also be permanent (though potentially not always active) and ideally provide static rather than dynamic buffs in order to minimize their cumulative impact on game performance.
The buffs should also be plentiful enough so that their collective weight is able to open up or augment builds that are currently considered to be off-meta or not viable for late-game content. They should also be tied to quest-content to give context for the buffs (roleplaying) and to keep old content relevant to all characters.
Another few examples of well-done quest buffs from Skyrim that we might be able to use as inspiration for similar buffs in ESO. This one is from "The Heart of Dibella quest":

As you can see, this is another interesting buff that is highly useful in some situations while being utterly useless in many others (especially in ESO where many enemies are monsters, beasts, and constructs).

This one is from reading one of Hermaeus Mora's Black Books and is mutually exclusive with two other buffs. It is less directly translatable to ESO, but you could modify it to be something like: "Applying the Chilled status effect deals 3000 additional Frost Damage" for a small but noticeable boost to an off-meta damage type.

This is an example of a buff that is too universally desirable, but if it were tweaked to be: "Increases your healing done to allies by 25% with Stamina healing abilities" it would be a large buff to a specialized healing niche and leave traditional healing balance unaffected.
In any case, by now this post is long and the general point is made. With a little bit of creativity, we could, at a stroke, breathe fresh life into some of the game's oldest content as well as provide some buffs to roleplayers and some niche, RP-style builds.