The changes to Marksmanship Hunters aren't a new thing, as Blizzard have changed the way other classes work in previous expansions. Now they're creating a new playstyle for hunter players which will be a more tactical approach to the class.
World of Warcraft was changing up class specialisations throughout all it's previous expansions, and the same goes for Battle for Azeroth. Apart from changing up specialisations, it will be adding new zones, features and increased level caps. Marksmanship Hunters are arguably the be all end all to ranged classes in World of Warcraft, but both Marksmanship and the remaining two specialisations have been in a downward spiral in Legion.
The first change coming to the class is going to be related to the removal of the Vulnerable status effect. What vulnerable does to enemies is make them receive additional damage from Aimed Shot. The new version of MM hunters will be more of a game of cat and mouse with the players' resources and cooldowns instead of Legion's basic spell rotation.
Quick Shot is an ability which will be changed from the current passive effect into an active spell which costs 15 focus. The change from hunters using mana to focus was made in Cataclysm, and the new update to hunters in Battle for Azeroth furthers the need for players to manage their resources as well as procs and cooldowns.
New talents include Trailblazer, which will be on the level 30 branch, and will increase the hunter's movement speed by 30% when not attacking for three seconds. On the level 45 branch the Steady Focus talent will decrease cast times of Steady Shot by 0.25 seconds each consecutive time it is cast, up to a maximum of four stacks. Casting any other ability will break the effect and players will have to start over.
Battle for Azeroth is mixing up how the class specialisations work in many ways for each specific spec. Be they good or bad, we cannot say as players are yet to fully dive in the classes, rip them apart and find the pieces which work well together.