[plamzi]

I think looking for a game that is challenging past the first year is not a hopeless quest, and I'm a bit surprised you haven't found any. You just have to look for signals that there are clear long-term goals and the gameplay doesn't pull any punches. And/or you can look for games that seem to be developed very actively - so that even if the gameplay is fairly easy, changes and additions compel you to look for new strategies.

I don't think any MUD creator tries to make things uninteresting to mid- and late-career players. It's just that some games choose to cater to more casual tastes, and it's generally a lot easier to provide short-term goals. When/if they do get around to thinking about long-term goals, many are tempted to extend the vertical axis (add 100 more levels, remort/multi-class) rather than the horizontal, which requires new content and is therefore harder to pull off.