Originally Posted by
Teriz
So after the Evoker, it appears that this is required for new WoW expansion classes;
1. A major lore figure to tie into. Death Knights have Arthas, Monks have Chen, Demon Hunters have Illidan, and it appears that Evokers have Alexstraza.
2. A playable version in MOBA/RTS format. Blizzard seems to favor players getting a preview of future class mechanics via their MOBA/RTS games. When the new class releases, the pull abilities and mechanics from those games.
3. A landmass that thematically connects to the new class. DKs had Northrend, Monks had Pandaria, Demon Hunters had Broken Isles, Evokers have Dragon Isles. The next class will likely also have a landmass that matches its theme.
4. Abilities that don't fit in existing classes. The expansion classes house abilities that simply wouldn't work with classes that existed at the time.
Obviously, none of this is set in stone, and Blizzard can do whatever they want, but at this point this appears to be the keys for future class implementation. If you look at the recent DR implementation, they seem to fail at #3 and 4, which partially explains why they weren't implemented.