The latest Diablo 4 cinematic trailer from Diablo 4 Gold Blizzard showcases Inarius fighting Lilith Armed forces at both of their backs. While the scene does not mention Tyrael's involvement directly, his design of the Horadrim and the influence he has on the various Paladins of Sanctuary implies that he's definitely involved. Light and dark clash across the field, but now-mortal Tyrael is a middleman.
His presence will be experienced through these intermediaries unless circumstances force his hand. Tyrael has a hard time coping with the aftermath of destroying his Worldstone at the end in Diablo 2. The mysterious artifact protected Sanctuary from Heaven and Hell and now acts as a protector for the region in its place. Tyrael does not appear likely to lead any further actions into Hell or venture much further beyond The Pandemonium Fortress, where he may offer advice like Deckard Cain for people who remain and listen.
Five playable Diablo classes have been announced so far and all of them are takes on options from earlier games. Given each installment so far has been different and exciting, maybe Tyrael could become a player in the battle and help balance the sides by providing players with the possibility to be transformed into angels or demonic powers to perform duties to Sanctuary. This kind of system could easily follow on the multi-tree advancement already revealed for the series.
It is unlikely that Blizzard will make Angels and Demons into classes, considering that the franchise centers those trapped between their ways. But as the mortal truly trapped between Heaven and Hell, Tyrael sits in a unique position ever seen in Diablo history.
It is likely that he will be playing a role of balance in the story, maybe even the voice of reason instead of taking sides like the past. Although he may be forced to buy Diablo IV Gold take sides, Tyrael now understands the true value of the mortals. That is why he'll likely take part in the events of Diablo 4.