TLDR - Put new, rare (ultra-rare?) NPCS and events in current zones that rarely and randomly patrol vanilla zones that hit like trucks and make things more unpredictable.
The newest weekly endeavor of "kill 20 bosses" got me thinking about how shallow world bosses have become. They are just static loot pinatas that get ganked by raving mobs of players at this point. IMO this is indicative of my biggest complaint about ESO in general, about how boring and predictable open world exploration gets. I think Angry Joe put it best in his initial review of the game several years ago when he called it "painfully average". The game has of course gotten way better since then, thanks in no small part to the devs' willingness to admit errors and try to improve things based on feedback.
I think a big part of this "painful averageness" has to do with the predictability of world encounters, and I think my experience so far killing world bosses for this endeavor has really shone a light on this aspect. Granted, there is *some* unpredictability insofar as there are random spawns already in the world (i.e. Werewolf hunters, random daedric portals etc), and the anchors can spawn different enemies... but they offer very little in the way of challenge or "true" surprise or rarity. They are just different "flavours" of the same farmable activity - i.e., a loot pinata to whack with your stick until the goodies come out.
What I think would truly shake things up in the open world would be truly challenging enemies (i.e. world bosses), or perhaps gangs of roaming raiders, true "player killers" which can take you by surprise while you're farming ore nodes and put an end to your cockiness for a while. The problem with static world bosses and static anchors is that they are so easily farmable by groups who aren't looking for a gameplay challenge, but who just want to slice through things like butter to collect their goodies at the end. (This is fine, I'm not complaining too loudly about that... This is a very normal activity for an MMO.)
But wouldn't it be more fun, unpredictable and challenging to have this randomized in the open world? Something similar to roaming dragons in Elsweyr, except way more rare, less predictable, with no map markers or anything... Kind of like the Fel Reaver in WoW BC classic. Something that would truly patrol by (not just "appear" and stand there waiting for you, as in current random encounters) and stomp you into bloody goo if you're not on your toes.
This could encourage random grouping and spontaneous cooperation, or perhaps even a new unexpected solo challenge for min/maxed players just doing farming runs out in the world.
(Just throwing this idea out there for funzies so all you naysayers can start writing me essays about how it won't work. I know there is probably a 1 in 1,000,000 chance ZoS would actually do something like this.)