It is going to come too late. Path of Exile 2 will gobble up any recurrent players it would have gotten. Path of Exile is everything D3 should have been. It has so much iteration and development already put into it, I have a hard time seeing how D4 can compete.
Blizzard made the same mistake with Heroes of the Storm. It was even better than League of Legends (LoL) in some ways (meta-reward structure, new maps/mechanics). A real shot-in-the-arm to a stale genre. Despite all their promoting, Blizzard still killed it because the player-base/money just wasn’t there. It did provide a lot of stuff for LoL/Riot to ‘steal’ though, so that’s nice.
So - in my crystal ball, I see Blizzard repeating what happened with Heroes of the Storm and Diablo 3. They’ll pump content / purchases into it for a couple years, then leave a skeleton crew to maintain it.
All that negativity aside, I am... hopeful for a fun 3-4 hour campaign and some dark atmospheric story-telling.