In the years following Inarius and Diablo IV Gold Lilith had secretly created Sanctuary for the Nephalem - - the result of both angel and man, the council of Archangels came together to decide the world's fate. Tyrael was the one who cast the final vote, and consequently, he was held accountable for the decision by others in the Angiris Council, leading to being mortal prior to happenings in Diablo 4 all for the for the sake of humanity. The role he plays in the next game is still unclear however there are possible ways that this key character could factor into the future of Diablo.
Diablo 3 is a deep dive into the story of Tyrael, who transforms into"the Fallen Star by tearing off his own wings before falling into Sanctuary where the descendants of the Nephalem dwell. By joining forces with a powerful descendant, Tyrael banishes the Evils of the First Order and takes control over his Black Soulstone. The moment the Archangel of Death, Maltheal, went after the stone, and then took on all of his allies He was the one who brought his partner to Pandemonium to stop death from happening.
Tyrael chooses to keep his mortal form . However, he is reunited with his angelic brothers and becomes both his Aspect Justice as well as of Wisdom. Diablo 3 ends with him looking to see if his new mortal companion, who could be corrupted, should be the next enemy in Heaven, Hell, and Sanctuary or life itself. Aspect of Justice and Wisdom Aspect of Justice and Wisdom likely won't sit idly by while Inarius makes his way through Hell in search of his past love, Lilith.
The latest Diablo 4 cinematic trailer from Blizzard includes Inarius confronting Lilith as well as armies at both their backs. While this footage doesn't include Tyrael's direct involvement, the invention of Horadrim and the influence he has on the Paladins of Sanctuary indicates that he's involved. Dark and light clash on the battlefield, and now-dead Tyrael sits somewhere in between.
It is likely that his presence will be felt through these intermediaries until events cause him to act. Tyrael has a hard time coping with the aftermath of destroying the Worldstone at the conclusion in Diablo 2. That mystical artifact protected Sanctuary from Heaven and Hell, and he now is the protector of the land on its behalf. Tyrael seems unlikely to lead any further journeys to Hell or travel any further than to the Pandemonium Fortress, where he might provide advice like Deckard Cain for people who remain and buy Diablo 4 Gold listen.