I can see the appeal of an ending, although that's not something that I personally look for in games. I think the biggest problem in designing this conclusion/ending mechanic is making it feel significant, but still allowing the player to come back later on (especially if there's an expansion to the game later.) MMOs really have an interest in keeping players playing, so I think any kind of ending is also going to have to leave the door open for players to come back, and probably even encourage some players to replay the game more.
Perhaps some sort of highscore entry could work? Each character gets one opportunity to "finish" at a point of their choosing and they see a cutscene and get a special in game title depednding on their achievements (lore wise, the idea of "finishing" could be the player retiring, or maybe it marks their graduation from training adventurer, maybe it's signified by receiving knighthood or induction in some special guild, or so on.) In any case, when a player chooses to finish, all their achievements are tallied and they can see how well they did according to various metrics: total wealth, all the dungeons they've beaten, PvP K/D ratio, and so on.
This way you could have a way to put a capstone on your character if you want to quit, but it would also encourage other kinds of players to make multiple alts: go for a highscore of fastest to max level, lowest level to beat a certain dungeon, make a pure crafting character, and so on. It could even use a roguelike inspired feature where each character that finishes would give the player's next character certain starting bonuses, or maybe some kind of account wide flair for your other characters.
A bit of a tangent, but as a side benefit to the roguelike idea above; one of the other problems this might address is that in MMOs that fall out of popularity, the population tends to get concentrated at max level endgame content, so players who pick up the game years after release don't have anyone to group with until they get to the endgame too. That was one of the problems I saw a lot of in Age of Conan where as the population dropped it became impossible to find groups for all but the highest tier dungeons; they eventually added some achievements but there was never really enough of an incentive for most players to collect them. Encouraging the hardcore players to properly play through low level content again would probably be good for the community in this case.