You can't really design content such that it's endlessly future proof, short of allowing players to do it solo once the next expansion is released. Players go in after the rewards when they're current, but the eventual drop off in interest is inevitable, especially when you move on to the next expansion.

I do think that 'Field Content Zones' and having a real relic progression system is important, however. If this forum is any indication, there's probably a subset of players out there that spends an inordinate amount of time in game despite being averse to even attempting midcore content. The lack of a 'grind' around the relic this expansion meant that such players had no way of distinguishing themselves from players who did the same content but who barely played at all. I do think you need to have a low skill but time-consuming grind present every expansion, otherwise you risk leaving such players feeling disenfranchised, which is probably reflected in a lot of the recent backlash in here.

I liked elements of Eureka's 'exploration' and stealth elements, as well as the sense of danger that you had exploring the map. I also very much enjoyed Bozja's duel system, although it would be more accessible if there wasn't a competition to attempt the fights themselves. Overall, there needs to be an interesting story, maps that you want to explore with a sense of danger, and weapons that are exciting to unlock and feel like something more than glorified tome weapons. It's unfortunate that ARR used up a lot of the truly legendary weapon names, because no matter many superlatives you add to your Amazing Relic Axe, it's just not going to be the Ragnarok.