Path Of Exile 2, despite the name, isn't technically a sequel to Path Of Exile. Instead, Path Of Exile 2 is best understood as a significant patch/DLC to the original game. If I tell you that its original working title was "Path Of Exile 4.0", perhaps you'll start to see the shape of it: this game started life as a major update to the original game, and indeed it retains a lot of that history, since Path Of Exile 1 & 2 will still be linked by a shared endgame. Furthermore, any in-game purchases you've made in Path Of Exile 1 will be available to you in Path Of Exile 2.
What Path Of Exile 2 will offer over its parent game is a new seven-act storyline to supplement the original game. It will sit alongside Path Of Exile rather than supplanting it, and it seems like it'll automatically be available to players who've downloaded Path Of Exile from Steam or the Epic Games Store, as well as PS4 and XB1 for console players.
