There should be more companions/origins in final release. Karlach seems like a sure bet due to datamining, and her content that is already in the game would support her being recruitable (tadpole, offering to join your party).
Are mercenaries already in the game?
If merceneries work the same as D:OS2 then it is essencially an ability to create more custom characters - as many Tavs as you want. At this moment you can do that by hosting multiplayer group by yourself, but there should be in-game option for that once the game is finished. In D:OS2 there was NPC that allow you to recruit (create) a mercenery after act1. In PoE1&2 you can recruit (and create) merceneries at any tavern.
