Edit: Thanks to OlyverW, I'm going to use a workaround for the character limit. I hope this is okay.
I think it would be a fun idea to allow players to undergo some sort of quest to experience the old world. Not in terms of mechanics, just in terms of going to the old "layout" and experiencing the old world's content, from bosses to quests and such. Obviously the old world had major problems, this would essentially just be a history lesson deal.
As for a story hook, I think the "echo" would suffice. Perhaps someone could tell your player that they need to see the final days of the 6th era for themselves, and uses your "echo" to send you into the body of an adventurer. As such, from a story perspective you'd "start" in that nation's original "starting cinematic" and go from there (though with your levels, equipment and such intact, making the early parts a breeze and get the players to that meteor questline as fast as possible).
You could "return" from this echo to the present day at any time, so players need not be stuck there, and it would be purely optional, a side quest. Players would only pursue it if they wanted to, and FCs only if they wanted to add old world raids to their rotation. Once the quest is finished, maybe some big reveal like "it really was you, you relived your past just now but forgot everything after the calamity", but maybe not, either way you'd still be taking part and seeing it all for yourself. After the "echo", the player can still "return" to that world any time they wish, not for story reasons really but just for the sake of keeping the old raids going. Yes, maps were much bigger, but there are ways to code the game so that massive areas only stress a system as far as the game actually renders at any one time (other modern MMOs like WOW manage this well enough).
Ultimately, I just don't like seeing past storylines die out, never to be experienced again. This would allow newer players like me to experience all that, and a good nod to what came before. Again, this isn't a revival of the old systems, the gameplay would function in a modern way, it's just old content I'm requesting. What do you think of this idea?