I've gotten through a few rounds of Endless Archive and I've had a great time discovering all of the new mechanics and rewards. As someone who doesn't often do a lot of group content, I don't get to see many of these bosses and so it's exciting to get to experience them (and defeat them!). I also love that it's been tuned for solo, duo or solo+companion play, as the latter suits me perfectly. Props to ZOS for coming up with such a neat idea.
Now, I must admit that I am an Endless Archive novice, having only made it to the second arc. That being said, after reading about the overall mechanics it seems sort of formulaic - travel through two levels, fight a boss, do that over and over. I'd like to see more variety in the stage design, with interactivity beyond waves of monsters. This is the perfect venue to add things like traps and jumping puzzles, mechanics which go beyond the bog-standard, ceaseless fighting. Adding these elements into a procedurally-generated level design could add some welcome spice to the otherwise rote combat we're all used to by now. And if that's too much for combat-focused players, stick it in the Portals to the Unknown and give more adventurous players the opportunity to take a break from the fighting.
I'd also really love to see something akin to a Slay the Spire map, which would allow you to plan out your progress through the Archive, rather than a simple "two levels of mobs and then a boss" idea. The current format is fine, but I just think it could be more interesting. And I totally understand that EA is very new, I'm sure ZOS already has a long list of ideas. I also don't expect them to deliver me the moon. Just my thoughts. Thank you ZOS!