Since getting into Arcana for the first time and completing the story, I've found that there are a number of things about the Spirit of Harmony NPC and Boss that I think could be changed.

Firstly, when you complete the storyline, all of the music for all of the maps (except for the five Labyrinthine Cavern maps) changes. I like both the pre- and post-storyline music pieces, but it's rather sad to no longer be able to have the pre-storyline pieces in any of the maps. Being able to change which tune plays throughout the forest via the Spirit of Harmony NPC would be a nice update.

Secondly, the Corrupt Spirit of Harmony boss seems very pointless, especially when compared to the other Vanishing Journey boss, Arma. Arma has a custom map of its own with its own monsters (Arma's Followers). It gives EXP like every other field boss and major boss. It drops a large amount of mesos (and possibly drops equips too). The Corrupt Spirit of Darkness, on the other hand, has a duplicate of the Arcana: Grove of the Spirit Tree map, in which there are no other mobs but the boss. The boss drops nothing, gives no EXP, and has less than twice the HP of the level 240 Discordant Spirit mobs farther into Arcana.

I would recommend the following:
Give Corrupt Spirit of Harmony a specialized boss map (not just a copy of an existing one with the NPCs removed)
Increase Corrupt Spirit of Harmony's HP to be at least 100 times Discordant Spirit's HP
Add Discordant Spirit mobs to the boss map
Give the boss a drop table similar to (but better than) Arma