A few suggestions after playing the game way too much over the last month.

[Plena]

-More sinful choices/actions outside of Steward/King/Council/Valar stuff. Make sin more rewarding in some ways but much more painful in others.

Examples:

1. Ability to kill helpful npcs like Treebeard, Hercules, etc. for greater stat boosts and massive sin. 
2. Ability to kill merchants (please I need to kill some of them so badly).
3. Special actions unlocked only at very high sin levels. Maybe something like being able to eat the corpses of players/monsters for stat boosts.
4. High sin makes degens a lot worse.
5. High sin causes permanent poison.
6. High sin has a chance of attracted much higher size monsters.

-Remove Balrog's experience drain.

This can be exploited to create characters with unreasonably high stats. Though doing so requires abstaining from picking up any special titles (King, Council, Valar, etc.), it's still a way to avoid having a character die. I have had some fun exploiting this, but I feel like it's outside the spirit of the game for such an exploit to be allowed.

-Allow Council Members, Valars, and Ex-Valars to find the Arkenstone.

It's a little silly that one of the pinnacle treasures of the game can't be found once you cross the level 3000 threshold. Is it a bug, or intended? Perhaps a few tweaks to it's benefits would make this a viable change.

-Allow the Lost Mines dungeon to show up at all levels beyond 300.

The Lost Mines is one of the coolest parts of the game, in my opinion. Why make it go away at 900?

-Add new dungeons.

Examples:

1. A gauntlet type dungeon where you go in a straight line, facing ever greater challenges. You can always leave after each fight, but the growing rewards offer incentives to continue. Randomize the length, with longer version being extremely rare (and difficult!)
2. A maze dungeon with no monsters and only a single great reward at the end. Similar mechanic to torches to force limitations on time spent inside.
3. A dungeon with some kind of logic puzzle. Solve it correctly for a nice reward. Get it wrong and suffer greatly (permanent stat losses, high poison, etc.)

-Add a secondary goal after 3000. The ability to become the new Dark Lord.

Maybe a high level character decides that the grail is for losers? Make it possible to find an evil grail with high enough sin and descend to the depths of depravity as the Dark Lord. Have this also change the name of the Dark Lord monster to "Dark Lord <Playername>"