If Bethesda is creating this, then they have to create some method for joining the Raider or Slaver faction to finish the game. So that means missions that requires joining up with the Raiders or Slavers or starting a Raider or Slaver gang, do a bunch of missions to increase your reputation with them, then take out the other factions. Radiant quests would be needed for the Raiders and Slavers. Raider HQ and Slaver HQ would need to be created since there is no Slaver HQ and the other Raider bases could be destroyed before you join them.
Then you have to introduce new companions since most of the current companions have a problem with Raiders and Slavers. Codsworth, Dogmeat, and Strong would likely not care about being a companion to a Raider or Slaver, but the others would. So that means that the same amount of work that went into creating each current companion has to go into creating each new companion. Also, the previous cast has to be brought back to record new lines for joining the Raiders or Slavers.
Then you have to add missions to deal with the new Raider and Slaver threat for the current factions. This is at the absolute minimum for creating 2 new factions. To conquer the Wasteland, it would be necessary to deal with all the different Raiders which requires more work. So either destroy them or unite them under your banner. The amount of work to do just this is far more than standard DLCs which add 10 missions to the game.
Then you have the problem that less people will buy a $25 expansion targeted towards Raider and Slaver factions. You are thinking too small if you think it is possible to fit your idea within a $5 to $10 DLC.
The other idea about policing the Commonwealth, establishing diplomatic ties with other groups, and establish policies is more likely to fit within a $10 DLC. Although, that would only work with the Minutemen and Institute since only the boss can establish policy and their is no way to become the boss or establish diplomatic ties in the Railroad and Brotherhood unless some changes are made to previous missions like
Spoiler Blind Betrayal where you or Danse challenge Maxson for control of the Brotherhood
.