Those newer to the game may have heard about how some quest bosses like Balreth and Doshia and Doppleganger Lyris used to be hardcore. Maybe you like some dungeons and trials mostly because of one boss, the highlight of the run.

Sooo...

Consider what it would be like to have these NPCs in a new type of instance, buffed up with some new abilities and mechanics and maybe some adds, that you can queue into. Just grab a few friends and jump in and there is your big bad boss ready to smash you into paste.

The dungeon/trials devs could have soooo much fun thinking up ways to revamp the bosses, similar to the buffing of world bosses (hey, maybe some of those could make an appearance). We would get new ways to experience old challenges and of course achievements and other rewards. Toss in some kind of insanity mode for those who need that kind of thing. Each boss could even get different version of itself over time so that you wouldn't go "***-hum that one again", perhaps even some random elements to it that combine to make different types of challenges.

The devs could start off with some limited number, and players could speculate over which ones are going to be added next.

The goal isn't to replace dungeons and trials, but to have a quick run challenge for PvE like a "battlegrounds". And to maximize use of existing assets to allow more creative experimentation from the devs to help better revamp old content and make new content. The current dungeon/trial gear drops and such still come from doing that longer content. Plus, most of us have our favorite fun quest bosses, mini-bosses, dungeon bosses, etc. and not all are repeatable unless you roll a new toon. Getting them buffed with new, expanded, or altered abilities and surrounding mechanics, especially with some randomized elements, also gives us more to do that isn't as routine. To iterate on this idea, there could even be a chance for players who wished to do a string of these bosses to earn challenge achievements.

I hate to give examples of what this might look like, because your imagination is better than what I can write and I don't want to limit the possibilities by giving something to "represent" those possibilities, so please take this as a the lowest level of quality possible for my idea. Imagine Balreth fighting like you imagine he would versus the Covenant invaders, with a huge AoE rain of fire that is forcing your healer to rapid burn their magicka to keep you alive. The DPS have to set sigils or crystals or totems up to summon the water rain magic to stun Balreth and increase the damage he takes (for the hard mode the rain to make him vulnerable to any damage). The scamps in the fight come to steal the totems which starts the fire rain again until the scamps are caught, killed, and the totems set up again but the scamps are fast and teleport. But Balreth summons skellies that go after whoever has the totems and their attacks interrupt the set-up process.

So that is a partial example of an enhanced Balreth fight, and you can add randomized elements to it like having Balreth use ice magic instead of fire to snare and immobilize players, or perhaps poison/disease damage to keep DoTS on players and make healing a bigger pain, etc. The sequence of certain attacks and phases could be changed. You could replace the scamps with trolls. These are, again, not the best of the best ideas, just example of how wide open the variations can be. To flesh out the ice version of Balreth a little, during his room-wide AoE phase everyone is snared (think of the quest in the rift where you had to light fires to stay alive) and sometimes frozen by a whirlwind (think of the start of Aetherian Archive) while the totems are set up. These activates a localized fire rain over Balreth to stun him and have him take more damage/be vulnerable. But now the whole floor is slick ice, and moving your character causes them to slide. So you have to chase then sprinting/teleporting scamps this way, with big AoEs dropping from the sky that freeze you in place (lucky and skilled players can try to pair these with the fast slide effect to get someplace then put on the brakes). Once you get the totems back the continual snare/AoE damage phase starts again to slow you down trying to get the totems into place.

The plot and lore for this could be Vaermina, with these enhanced versions and weirdly modified versions of boss fights being terrors that she manifests from/within her nightmare realm of Quagmire. Some locus or intersection with her realm has been found in Tamriel, though maybe NPCs don't know that's what it is, they only know the rumors of the insane horrors found in that haunted place...