I submit here what I hope is a relatively simple change. I'd greatly appreciate it if mobs significantly lower than your level - low enough that you would get no XP or loot from their defeat - were less inclined to aggro to you.

Running through an old zone to get lorebooks or skyshards you missed becomes a hassle when mobs are still keen to rip you apart, even when they don't stand a chance to do so. Players don't generally target creatures significantly above their level, why should enemies?

I recently returned to the Alik'r Desert to pick up a few things I'd missed before (lorebooks, Lesser Circle credit, etc.) and at one point found myself swarmed by seven Ra-Netu, who happen to aggro incredibly quickly - even sprinting directly away from them when I hear their notification isn't enough to get them off my back. When the smoke cleared, all I'd gotten for my effort was the distinct feeling of wasted time.

There is, of course, some risk to this. I do every quest I find and run each dungeon in each alliance as I progress, so I often find myself overleveled for even new areas. Even so, I usually need to kill the enemies for a quest anyway, so I'll still get into combat... I also only propose a reduction in how quickly the mobs become aggressive, not an outright removal.

Also, as an early counterpoint: while Stealth is an option, it isn't guaranteed to work, and the decreased speed just makes it harder to avoid aggro after being noticed. (And why am I trying to sneak past enemies I could wipe the floor with, anyway?)