I have a feeling that inarius will be rebirthed as a new incarnation of hatred that we potentially saw in those flashes after the credits. Between him and Lilith he was the one who had true hatred for sanctuary whereas Lilith only had (maybe) love of some sort and only seemed to hate the eternal conflict. Also in the prophecy Lilith is also explained to be pierced by a spear of light well, you can take that very literally as the spear inarius held or (as he belonged in hell and the spear was held with greed/hatred more of a darkness spear if you will) she is to be reincarnated in heaven as an archangel of some sort. Just my thoughts I think it would have very cool implications if they swapped places in the eternal conflict.
Definitely a possibility. Based on what we saw with Neyrelle and Mephisto, I’m guessing the next expansion Neyrelle will get corrupted by Mephisto. She will either become his minion or potentially stick her soul shard into herself. Either way Mephisto will be an upcoming boss.
Before Neyrelle captured him in the soul shard, he was directly saying that capturing him in the soul shard would destroy sanctuary. What I think he was omitting was that he would be the one to do it.