A fierce warrior introduced in Diablo 2 and recently rediscovered in Diablo 2: Resurrected, the Amazon does share similarities with Diablo 4's Rogue, but it brings something unique to the table via the javelin. Another archetype that could find itself at home in the lands of Sanctuary is the Amazon. For these reasons, considering that none of the existing classes comes close to the playstyle of the Paladin and the Crusader, this is the most likely option for a sixth Diablo 4 class - but not the only one. With Diablo 4 putting a lot of emphasis on the live-service model Blizzard has been working on, it seems reasonable to expect a new class to come alongside major story beats. Since the series is all about defeating demons and protecting the land from evil, this is not only a fantastic choice in terms of different playstyle compared to the existing Diablo 4 classes, but it also makes a lot of sense from a lore standpoint and could even be used to expand on seasonal storylines.

One of the most likely candidates to become Diablo 4's new class post-launch is the Paladin or Crusader, an archetype that was first introduced with Diablo 2 and then found a lot of success with Diablo 3.

Diablo 3 crusader forums