Diablo IV can stand on its own as a traditional game tho... unlike Destiny. Or should I rephrase that, it's a much better standalone game than 1st year Destiny's.
As a long term GAAS tho... unless Blizzard has something amazing in their pipeline... that ain't it chief, and they got at most a full year to reel folks back in with something special.
Blizzard tried to copy the new enemy type trick that Bungie was so successful at with the Taken. In this case, for Diablo IV it's the malignant, akin to the Taken as a relatively cheap asset swap trick that tries to trick the player into believing there is something new and fresh to fight.....that is also easy for the devs to churn into fast content. The execution however - total novices .....night and day difference between Bungie and Blizzard.
Should have done something with the angels and an angel invasion instead of malignant beasts and hellspawns. Say have an angel, sort of a mastermind type come down to "save" the land (in reality invading) and fill Inarious shoes where Inarious failed. The method of this angel to reach his goal is subjugating the land and brainwashing humans through raw, magical possession, no compromises. The angel's mean of possession, this power, is not that of faith but raw and possessive mind control.... akin to Lilith's corrupting power but with a magical touch, of a magical nature. The angel and the "invasion" arrives as a surprise because it's not believed Angels could operate or be willing to operate in such a manner so it creates the scene of instant shock. Instant shock due to the unreadiness and powerlessness to defend against it - initially obviously. That's a good theme to always have to make the antagonist all powerful and otherworldly - big power surprise with no quick answers - Bungie does it all the time - and it works.
So this angel will enlist his army through forced conscription and magical possession. The new enemies being mostly human type: like monks, druids, knights, non-corrupted animals etc... and they would get an asset swap of sorts with white eyes n glow - or whatever the art team can do to make them look different while being the same assets = faster content production.
The clash with the player, the wanderer, is inevitable as the player is forced to confront it, fleeing being just a temporary recourse at the very outset to ensure survival. The choice presented to the player will be to find a way to ward off the magical power of possession first and fight later. All throughout the player will avoid being possessed himself. The wanderer is obviously on the back-foot as village after village falls and he rather neither confront the angel or kill humans possessed.
After finding a successful way to ward of this power of possession then the wanderer and his companionship begin to device a way to start fighting and eventually kill or banish this angel back to the heavens. The tension is thus created by the fact that the wanderer and Lorath must above all protect themselves because if the secret that Neyrelle is in possession of a prime evil spills from their brains it would be of most interest to this angel... who would undoubtedly search for Nyrelle. This angel would also find out exactly what happened surrounding Inarious death. The angel in some form or fashion at the very late progression of the storyline gains hold of Lorath, takes possession, learns of everything, and that knowledge becomes common knowledge in the heavens. Lorath either dies at the hands of the angel by trying to fight the possession (preferably for that gut punch) or is freed by the wanderer as the wanderer finds a way to defeat the angel. The angel is not defeated through death in a strict sense but by a banishment ritual, which takes place in the actual battle with the angel itself and is the culmination of the raid-dungeon fight. Banishment is preferable to death cause you can always write something to make that antagonist come back in some other season or storyline and bill the pitch as "finishing the fight".
That would have been much more effective - the angel being a raid-dungeon boss, in a cooperative capstone dungeon on steroids for hunting unique's. This capstone dungeon would feature puzzles, preferably for a co-op party of 3 (including the player). The dungeon would have a story-only version, where the puzzles are completed with the help of companion npcs strictly for story purposes. After the story is done, it's a co-op capstone raid dungeon with actual loot - and obviously a bit harder to complete. Makes it easier to design something with 3 players/npcs max involved. Obviously as part of the expansion several new regular capstone dungeons featuring the angel's underlings and minion bosses should also be added. Each chief minion underling having their own, respective capstone dungeon - a chief monk, a chief druid, a chief knight, a chief beast. 3 or 4 should be enough.