There’s a set story, but it’s discovered. The world is wide open, and the player can go anywhere right at the start of the game. There’s minimal railroading at any point.

Unless I misunderstood what you meant by emergent narrative. The progression through the game requires the player to learn what to do by interacting with aliens and also exploring a bit. There is an in-game hint system (an alien dialogue tree with prices), but there are often multiple solutions to each “problem”. The player can even get through the game being good or evil – whatever they choose!

The game plays very differently than ME, but you’ll probably find the dialogue trees very familiar. And I think SC2 actually does them better than ME.