WoW isn't really the type of game that would benefit from monthly updates, though.
I used to play Asheron's Call. They have had monthly updates, as far as I am aware, since 1999. Those updates include new quests, new items, new enemies, and new dungeons. However, most of the new NPCs are just reskins/renames of other NPCs (with updated stats), the dungeons reuse textures and assets like crazy (they're basically a big maze populated with respawning enemy NPCs and the occasional quest NPC), and the quests simply involve plopping a few new NPCs in the world and having players complete easy objectives (kill something until a note drops, or kill x amount of mobs, or talk to this guy, or blah, or blah). In other words, the content is simple and easy to develop, especially considering this is an MMO from 1999.
None of that translates to WoW. The quest system is 100% different, the level system is 100% different (one of the main purposes of questing in AC is to gain XP, which you continue doing even after max level. XP is an incredibly valuable resource), and the dungeon system is completely different. The resources required for quests, dungeons, and items in WoW are much higher because the quality of the final product is expected to be much higher. Furthermore, quests aren't a very highly desired thing in WoW after reaching max level.
Long story short, there's a very limited desire for the kind of things they could offer with monthly updates. The quality of the product would suffer. I don't see something like this ever happening.