A lot of stuff is unconfirmed right now, but we know they’re taking a 7.3.5 Legion client and changing it to work like it’s a 1.12 client only with all the security updates they’ve had over the years.
Somebody made a thread covering everything that’s been actually confirmed by Blizzard, rather than hearing about it from people who are inherently biased.
Other than that, there are a few inauthentic things on the table right now like the 2 hour BoP loot trade window which is restricted to raids only.
Progressive itemization has neither been confirmed nor denied. They have only said they will start off using pre-1.10 “Drop rates and locations”. They didn’t say anything about stats.
The biggest concern brought up there is getting an authentic database of the original itemization. We don’t really want the guesswork that the private servers do. Online archives don’t cover everything like proc rates on weapons.