This wouldn’t alter the story of the game, it would fill the void between acts and be a nice ground to add more dept to the lore. Cain explaining the Mage clan war while travelling to our golein, telling you about Pandemonium or kurast
Between act 1 and 2 you take a big trip between the rogue camp to Lut Golein, what happens during this trip? You could discover areas between those 2 places, defend the caravan, encounter people requesting help
Same for between act 2 &3, you could encounter islands where the ship docks to refill
Same for between act 3&4 and 4&5, rather than the portal porting you directly where you need to go, you have some areas to walk before getting there
When you step out of the Durance portal you spawn at the gates of hell and you have to walk to pandemonium which is, according to the lore, is a crossroad, not a deadend
Then when you take Tyrael portal to harrogath, you could be spawning in a temple at the base of mount area, Harrogath being an outpost midway Up
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This kind of expansion wouldn’t add much lore but could be a ground to add more monsters, areas, game challenges and to deepen the heroes story
Use those interActs as alternative gameplay
“Defend the caravan from the ambush” waves of monsters come at you
“Cleanse the island” kill all monsters so Meshif’s crew member can gather food or repair the ship
“Destroy Diablo’s soulstone”
Note: it could be Hell only, creating a sense of novelty to the boring repeatetivness of norm/nm/hell and avoiding making rushing longer and over leveling