I think a predictable but well executed story is better than an unpredictable but messy one.
To cite the Torna example here, Torna is about an event that happens before Xenoblade 2. However, Torna is not just about that event. The actual event in question isn't until like the tail end of the game really, everything else is just exploring how the world was at that point in time, something the main plot doesn't really tell you.
Another example, Clone Wars. When you watch Clone Wars, you know how it ends. There is only one way it can end: Revenge of the Sith. And yet Clone Wars not only goes into new territory set within those three years but also tells some really good stories despite it only being able to end one way. There's a story arc where a Clone Trooper discovers the Order 66 plot. The audience knows he won't succeed in warning the Jedi, because that's how the story goes, but it doesn't stop the struggle, the suspense and everything else from being damn well executed.
In general, i do think that's the key with prequels. You gotta show that other stuff happened during this time, not just the Big Event.