In Seasons everyone starts from Zero, like a brand new account.
For more Details about "What are Seasons" see this News article for Season 8 start: What are Seasons?
After Season all Characters, Progress, items, new recipes etc will be added to your SC/HC normal Progress. Paragon exp will be added to your existing. Keep in mind that P500 normal + P500 season wont end in P1000)
Detailed Information about "What happens after Season" in this News article: Rollover after Season end
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It's mostly attractive for those of us who have played non-season a fair bit and would rather enjoy the build up again then the long term play of refining gear.
It depends on your own progression, and how much time you have and how much you enjoy the grind from level 1 to being able to do a Greater Rift at 70 or something.
If you are interested you should just try it once and see how you feel about it. Everything you collect on Season will get transferred to non-season after the season is over, so it's never wasted effort.
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Before last Blizzcon, before they announced what they had planned for Greyhollow Island and before they announced the Cube, I was convinced we were getting an expansion. 6 months later I'm far more pessimistic.
They've done artwork for several seasons worth of portraits. They've done other cosmetic stuff too, like wings. It doesn;t take much effort to continue the current content. Item here or there, some bug fixes, slap some balancing changes on some stuff, and you can do a season reset and basically have people entertained again for 3 or 4 months. After the cube patch there's really not much they have to add.
Between paragon, getting Ancient items, legendary gem leveling, Caldesann's leveling and the season reboot every few months there's something for everybody to aim for that is within their reach, and within their playtime scope that thay can do to progress and have fun. There's not much that needs to be added to that. In fact putting even more layers of progression on top of it is probably making it too much. The Caldesann's thing was already a really long term, high end goal they introduced that most of the players will hardly touch.
So the current state of the game is that it is set up to work well long into the future, and even if they added nothing further apart from balance changes the game would continue to draw players in every season/patch to play some more and have a perfectly good time.
So what does that mean for the future? The game doesn't need much technically, but there's also the story. We're actually at a perfect spot in the story to do a reboot. All the Evils are free again, cause of Malthael. It would make for a bit of a strange turn to now suddenly fight Diablo again, for the second time in this game's life. And who else is there? Who is a big enough villain or threat to realistically both threaten our Nephalem Hero's AND not infringe on the story of Diablo being the big bad evil of the franchise. The only remote threat there is is Imperius, should he feel threatened enough to start an attack on us. But frankly I wouldn;t like that story at all. Imperius may be a grumpy asshole, but he;s still an Angel, and he's still on the relatively good side of spectrum in Sanctuary. I'd hate to have to suddenly start fighting Angels.
And what of our Heroes? How much further can they grow lore wise, they are already peerless, even among their nephalem kind, at some point it's just not realistic anymore to have them continue to become more powerful. At some point you need a reset.
So storywise we are at a perfect place for a reset, and the game is set up to continue to be fun to play and engaging longterm, with minimal development needed. And then you come to the dagger, the point that is most telling to me: We've not heard ANYTHING about a new expansion. There was no hiring being done, there's been no hints by Blizz that there'd be a continuation, there's been absolutely nothing. In fact the one thing that Blizz has done that points to anything is that they've started hiring for a different game in the Diablo world. All of that is combining into a sobering conclusion that Blizz is probably not going to make an expansion for Diablo 3.
I'm pretty sure we will hear something at Gamescom this year or later at Blizzcon perhaps, but given all I wrote, I do not think it will be an expansion. I think it will probably be Diablo 4, with D3 continuing to get small content updates throughout development of the successor, but in most ways being finished.
I really hope I am wrong though. I'd love an expansion, cause I love my Heroes and I'm not sick of them yet and won't be for a long time. I hope their story continues.
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I'd settle for a system that worked just down right now. I can imagine Blizz not wanting us to farm endless crystals and turn them into Souls just for crafting, but if the system only worked down we'd at least have a decent way to get stuff we need, while still always aiming to get the best stuff available.