Tobold made a rather insightful (as always) post called Games for explorers in which he observes that most MMOs cater more towards the Achiever end of the Bartle spectrum. It would seem to be the case; its much easier to add new levels and tiers to "achieve" than it is to create genuinely new content.
Is it possible to have an RPG thats more Explorer oriented? In single player games, it was, since the leveling was more of something that just happened along the way.
I think there are some changes that could be made to improve future MMORPGs for Explorers.
One impediment to true exploration in MMORPGs is the static nature of the game. Everything gets spoiled eventually, and even if you try to avoid it, people tell you. And, since there's an Achiever in all of us, many of us eventually give in and look up on the spoilers sites to find out what we have to do. Half the time, it wasn't very interesting anyway.
I remember being excited that Dungeons and Dragons Online had traps. Real traps! And secret doors. And someone had to detect and disarm the traps and find the secret doors. In theory. In reality, everyone had already been grinding the same dungeon fifteen times before I came along (I was distracted by previous shinies) and they rushed us through, already knowing where to click and what to do.
While completely random dungeons are boring (sorry, City of Heroes), static handcrafted ones become boring once you've done them once. What we need is some middle ground where the static, handcrafted content has some degree of randomization to keep it interesting.
I would also like to see more puzzles - the type of puzzle that plays like a mini-game, not the kind you can look up on a spoiler site. Explorers like to learn about the game mechanics, and mini-games give us more mechanics to explore. It also helps break up the grind, and is much more interesting than pushing a button for a random chance at success. (An example would be the Pipe Dream-style hacking mini-game in Bioshock.)
Also, what do Explorers do when they've seen everything? Well, most likely, they move on to a new game. But why do developers insist on always creating entirely new zones when they could have parts of the existing maps change on occasion. A cave might cave in revealing something new. Change EXISTING zones more often instead of only dumping new zones into the game.
Let's make it possible to actually end a quest. Not just for ourselves, but for everyone. You know, so the NPC actually walks home and says "thanks guys" and stops needing the item. Things need to change. Have the leaders of a city be assassinated never to be seen again, and create new quest lines (in the existing cities) that let us explore what happened. Not all new content requires people to sit and create new art assets for months on end!
Maybe some day we'll move beyond the Achiever-oriented tiered level-and-loot based gaming. At least for some niche based game.