I'm not following the actual diablo plot, I'm just saying it would be more interesting than what we actually got, which was just a big "congratulation, nothing you did accomplished anything". Convincing a prime evil would at least be threading new ground.Um, what?
You can pull a "technically" and say that the Prime Evils have lost against humans in a sense, but that's one hell (no pun intended) of a technically. Dark Exile? Sure, the Prime Evils were each imprisoned, but all of these things ended up furthering their aims (e.g. Mephisto corrupts all of Zakarum), and that was after centuries of carnage. D1? Sure, Diablo is defeated 'technically,' but at the cost of the heroes' sanity, plus all of Tristram. D2? Sure, each Prime Evil is defeated in a sense, but it avails humanity nothing but a short repreive. D3? Yes, Diablo is defeated at the end of vanilla, but that's rendered moot by the end of Reaper of Souls. The entire thematic throughline with the Prime Evils (heck, Hell itself) is that they can't be defeated, that all you can do is buy yourself time, effectively. Every attempt to even seal the Prime Evils in soulstones has ended in disaster.
Even in the scope of D4 itself and nothing else, the Prime Evils have "won," at least in the long term. As I understand, it's one of the key drivers for Elias's motivations. Eventually, the Prime Evils will return, and when they do, there's nothing to stop them. Mephisto (as the Bloodied Wolf) outright states that he liked the world as it was (bleak, shattered hellscape) before Lilith turned up. Lilith is the threat to Mephisto, humans are nothing more than playthings sans their nephalem birthright, and that seems to have faded into the background).
I'm assuming this is based on POD-theory; as in, you're entertaining a hypothetical that would have to emerge from the ending of ROS, since none of that is true in D4 itself, or in anything between D3 and D4. Humanity lost 50-90% of its number by the end of ROS, there's nothing in-game (that I've seen) to suggest that technology has improved. If you want to suggest that's how the story should have gone, okay, sure, that's a different issue, but there's no way D4 could exist as is if it did. You'd need a completely different game for it.
Alright, seriously, what's wrong with Neyrelle?
Eh, could work. That being said, that comes more off as a repeat of Reaper of Souls.
As far as Neyrelle, none of your ally are ever useful and all they ever do is just weight you down the entire game. Ironically the only ally that's ever actually useful is Mephisto, hence why he's the only character I actually liked in the game.