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If you play a game only for the story, I feel sorry for you. For me, gameplay is what matters most in a game. To be more precise, the story is important in RPGs – it's the foundation there, but in strategy games (whether real-time or turn-based), it's just a backdrop – nothing more. The fact that you "fell for" something just shows that you didn't read the information carefully. The announcement that the game's action would take place in a different world was made even before E3. You can't release "Diablo 3" without Diablo simply because the game has that name. If the new installment doesn't feature Baal and Mephisto (as if they never existed), but has exciting gameplay and great visuals, 90% of people won't care and will be right to do so.
Besides, it's better to create a new world honestly than to come up with a stupid cataclysm like in the fourth game. You simply can't imagine a more degenerate justification for changing the gameplay – it's the pinnacle of absurdity. Therefore, instead of coming up with nonsense (something like "after the cataclysm in the world of the third game, half of the people teleported to the world of the fourth game, and the other half to the world of the fifth..."), Ubisoft and Nival are creating a new, internally consistent world.
Mr. Van Caneghem has withdrawn from the game. He was offered a position – he declined. That's it. The Heroes series will continue to develop without him.