I would like to see Nexus: The Kingdom of the Winds open-sourced. It was my first, and favorite, MMO during its hay-day. The combat was simple, but blazingly fast paced, and the rich lore of the korean mythos gave it such deep character. The community really delivered in that game, with most events being acted out by GM-run npcs.

There were poetry contests, and a lot of deep RP. The ceiling was endless -- once you got to max level, you could still use experience, for buying stat-points and hp/mp, which unlocked higher-tier caves to explore.

Subpaths were made by the players, and they got to work with developers to figure out their game mechanics (new spells, animations, hidden areas)

The game had actual harsh death penalty -- most of your items fell to the ground when you died, and after about 10 minutes, they became world lootable. Some items broke permanently on death, to keep up their rarity.

Also, the game didn't drag you along and coddle you. No such thing as a quest log, or map way-points. You had to know your mission. Some quests required actually saying (via the chat bar) the correct phrases to initiate conversation with them, which you gleaned from riddles or lore.

Anyways, the game was eventually sold to some company whom then marketed it to kids... completely ruined the dynamics of it, but it had been on the decline anyways. I would love to see it open sourced, so that I could host a public server in the spirit of the original!