From my history of gaming I've seen a pattern for what features keep a game alive and popular decades after being released. I've listed these features below:
1) Random Game Generator – Excellent it appears KeeperRL already has this feature. This allows players new experiencesadventures.
2) Game Editor – Being able to create your own world, treasures, puzzles, NPCs, cities, enemies and events allows gamers to essentially create a world for others to discover. It's important the game editor be simple for using… such as the editors from AoW:ShadowMagic or Heroes3. This allows more than just allowing players to discover a dungeon which has been created.
3) Game Modding – I'm generally not interested in modding, however this feature allows a game to evolve in ways otherwise not possible. The gaming community also works together for solving issues by using modding tools/abilities. Since you're the creator of the game you can decide which pieces are easiest for modding.
For example if you allowed modding then anytime someone complained about the AI or the graphics or the items, then the gaming community could respond with advising them to work on improving it.
4) Late Game Planning – Every game during the late game stages should have a possibility of an extreme event, rare enemies, earning a title recognized by the realm, new mysteries, and hidden treasures. These late game features are jewels which will keep gamers playing again and again.
~P.S- I've been updating my post inside the Bestiary thread with more ideas.
Best Regards,
NTJedi