Something like completely restructuring a majority of the combat aspects of a game is a huge project. They're adding to the class system, they have to balance all the classes, enemies, quests, leves, behests, etc... to work with the new system, create new content based on the new system and then of course test/fix/test/fix until it all works properly.

We don't even know all of what's in 1.18, just that it contains more of the battle changes.