In short, it is apparently a move from 4th edition to 5th edition Dungeons & Dragons. Developers have stated that the next module will introduce the biggest expansion ever along with the increase of level cap. They have changed classes in the past but now they are doing much more fundamental changes than ever before. They are even renaming all of the classes. For example, Great Weapon Fighter will become Barbarian.
There is a lot to cover but one of the changes is that they want to bring back healers. Since the launch of the game, in endgame dungeons one second you are at full health and the next second you are dead. Developers want to do something about that and make healing great again.
Here is a list of some changes. The actual list of changes is going to be huge.
* Level Cap to 80
* All classes renamed and completely reworked.
* Paragon classes reworked.
* New class powers. Old class powers distributed differently between paragon classes.
* Feat system and companion system considerably reworked.
* Recovery, Lifesteal and Regeneration removed
* Accuracy and some other stats introduced
* Offensive vs Defensive roll system introduced
* Four new zones (Halaster's Undermountain)
* Rank 15 for enchantments and runestones.