We already have content coming in sub patches, might as well go all in and have it officially scheduled that way.

Something like every 1st Tuesday of every month we get new content:

1st month (Main patch) - Main story and raid
2nd month (x.x3) - Relic related content, deep dungeon stuff
3rd month (x.x5) - allied society, PvP stuff
4th month (x.x9) - Variant dungeon, new stuff etc.

contents of the sub patches could vary and switch around but the point it with new content every month, this could not only provide a steady schedule, but may entice people to log in more.