Many of the MUDs I've worked on or played have had a basic aggressive/not-aggressive system. Adding faction-based aggression might add an interesting dimension (i.e. maybe Orcs are indifferent to you unless you've killed >100 of them, making you a famous villain to them).
Allowing characters to build castles or strongholds and defend them, or to take/defend in-game strongholds or resources could be interesting if done right. Dark Age of Camelot had an interesting system in that your "side" could get bonuses if they controlled more strongholds than others.
Automappers are often appreciated by players. Advanced MUD clients like ZMUD already have them but they don't always work well. If you have one built into your game engine you could be creative with it.
A scarcity system could also be interesting -- have prices drop on items that are commonly farmed, while the value of items that are not commonly sought out by players goes up. This will encourage players not to always camp the same items.
It's not a new idea, but a "trophy" system where a player gets less experience when they repeatedly kill the same type of creature could encourage people to explore more areas. Most of the MUDs and RPGs I've worked on have had players settle into a comfortable safe area and not bother exploring if they can level easily in one place.
Get rid of Midgard but build a good tutorial zone. Most stock MUD codebases use some version of the "Midgard" zone for players to start in. Having that zone gives veteran MUD players a sense that you're just "another stock MUD", but it's also daunting for people to learn a new game, so give players the option of starting in a "newbie" zone full of tutorial info when they create a character.
If you're doing PvP, it would be nice to have a bounty system: If players annoy one another, let them put a price on each other's head and the other players can take them out for a reward.
Keep score of lots of things and rank them. Every player wants to be the best at something. Maybe Ralik has killed the most Trolls in the game while Elgarin has cast more fireball spells than anyone else. Many of the people who play MUDs are drawn to statistics, so being able to present things that they have done in the game in interesting ways will keep them interested. This has manifested itself in "achievements" in the various online game scoring/tracking systems.
In the vein of the paragraph above, keep track of the quests, zones, or mobs that a player has finished and let them know how much progress they have made toward conquering the game.