A long time after Inarius and Lilith had secretly created Sanctuary with the D2R Items - the son of angel and human, the Council of Archangels was formed to decide humanity's fate. It was Tyrael who took the vote that decided the fate of humanity, and this means that he's accountable for the decision by others in the Angiris Council, leading to being mortal prior any events in Diablo 4 all for the for the sake of humanity. The role he plays in the next game is not yet clear However, there are probable ways in which this character could factor into the future of Diablo.
Diablo 3 explores the depths of Tyrael's tale, as he turns into The Fallen Star by tearing off his own wings and plummeting to Sanctuary with the descendants of Nephalem reside. In joining with one of the powerful descendant, Tyrael slay the Primitive Evils and gains charge of the Black Soulstone. In the event that Archangel of Death, Maltheal, went after the stone and turned on all of his friends, it was Tyrael who took his companion to Pandemonium to destroy death itself.
Tyrael is able to keep his mortal form but returns to his angelic family to transform into an Aspect of Justice and of Wisdom. Diablo 3 ends with him contemplating whether his new mortal companion, who may be corrupted, will be the next danger of Heaven, Hell, and Sanctuary or even the life itself. There's a reason that Aspect of Justice and Wisdom probably won't stand by while Inarius is a mighty army that marches through Hell to find his former love, Lilith.
The latest Diablo 4 cinematic trailer from Blizzard is a scene of Inarius looking down Lilith Armed forces at both sides. Although this footage doesn't mention Tyrael's personal involvement, his creation of the Horadrim and involvement with the Paladins buy diablo II resurrected items from Sanctuary implies that he's definitely in the game. Light and dark clash in the battle, while the now-dead Tyrael is somewhere in between.