It doesn't really need to be decoupled from levelling or discourage being ran repeatedly, but it doesn't make sense in a dungeon format because the playerbase for dungeons is a combination of people who don't want to do harder content, for whatever reason, and then people doing doing it for the rewards and in both cases, neither of them are really interested in wasted time in a dungeon scenario.
As for content that would reward exploration, the answer is honestly something along the lines of isle of thunder from WoW/any gw 2 map. Think a zone along the lines of say, anemos or pyros. Something with a lot of weird architecture and nooks and crannies. Fill it with random treasure chests that spawn out in the world, and also rare tough monsters that you probably need to get a group together for. Cram it full of jumping puzzles for no reason, disable flight. Put actual good stuff in the chests and off the rare spawns and there you go. People are incentivized to wander around constantly. Still sounds boring to me and just like in Eureka, I'd sit at the home base afk waiting for call outs, but there it is. Content for the wanderers out there.
If they really wanted to get fancy with it, and god help us they shouldn't because the coding that exists for this already aint great, they could have unique mounts with mobility actions that you'd need to use to get into places, kinda like how gw2 does mounts. So like a glider that you have to do a bit of platforming up a mountain to then be able to glide into a neighboring volcano for example. Or another mount that double jumps which allows it access to a few cave systems that are otherwise unreachable. Hell an underwater mount of some kind that is the only way to navigate currents into buried ruins or something. Make the actual act of exploring the central mechanic and not necessarily the combat you engage in.
They could still populate it with fates or whatever too honestly.