If we look at the situation today though, the easiest thing would be to remake some talents in the MM spec and replace others with the ranged Survival core abilities we had(as they were prior to Legion).
Basically looking at the MM talent tree, you could have something like this:
-Level 15 talents-
You already have Serpent Sting present here.
The Master Marksman talent - Aimed Shot reduces the focus cost of your next Arcane Shot/Multi-Shot by 100%. Just add Explosive Shot to this as well, in case you don’t want to use Serpent Sting.
-Level 30 talents-
Explosive Shot is there, rework this talent/ability into being an ability that replaces Aimed Shot when picked. You get Explosive Shot as it used to be but with 2 charges(up from 1) and have a recharge time the same length as Aimed Shot currently has.
Explosive Shot is an instant cast ability, as opposed to Aimed Shot that has a cast time and req you to stand still when casting it.
-Level 60 talents-
Can esentially remain as they are.
-Level 90 talents-
Have the Double Tap talent reworked to include Explosive Shot.
Basically this: Your next Aimed Shot will fire a second time instantly at 100% power without consuming Focus.
If Talent: Explosive Shot is chosen, this will cause your next Explosive Shot fired to have it’s duration increased as well as having it’s focus cost reduced by 100%.
…or your next Rapid Fire will shoot 100% additional shots during it’s channel.
-Level 100 talents-
Here the best thing would be this(In my opinion):
The talent Lock and Load should be removed and made a baseline passive effect for the MM spec. AND, while the effect gives your auto shots a flat 5% chance to proc. This should also be added to be a part of your Mastery Bonus level so that the more mastery you have, the higher the chance to proc this effect would be.
And in the place of Lock and Load, you would now have the talent/ability Black Arrow for those that would want to use it.
On a personal note, something regarding Black Arrow as an ability you could add that would make it slightly more fun to play with would be this: Every time you use Aimed Shot/Explosive Shot, the cooldown of Black Arrow is reduced by 2 seconds. With Black Arrow having a base CD of about 30 seconds.
This last part is just something I would like to see. It does not have to be there but could be fun.
Note
The passive effect Precise Shots currently tied to proc with Aimed Shot. You can just include Explosive Shot here as well.
The same goes for Multi-Shot as it currently works for Marksmanship where if you use it, it can proc the passive effect: Trick Shots.
You would now have it do this: If Multi-Shot hits 3 or more targets your next Aimed Shot/Explosive Shot(if picked) or Rapid Fire will hit up to 5 additional targets for 50% damage.
Again, While I would prefer if we had a ranged Survival spec separate from the other specs, the above suggestions I guess would require the least amount of work and you could still get a playable version of the old Survival.
A big upside if you do this is the fact that you would now have a way to play MM without any movement restrictions at all. Again, just my opinion ofc.