Players choose what they do in a sandbox.
Why are you trying to move the goalposts? ETA: Specifically regarding bringing in the trad vs neotrad debate. That would just be a distraction.
I don't consider my campaigns a sandbox. On the other hand, I populate the world, set up factions with their own goals and agendas, set up an intro session or three and then let the players decide what they're going to pursue. When a particular chapter/story arc ends I ask them what they want to do next. I repeat the rumors and possible goals, allow them to suggest something interesting I hadn't thought of. After they choose a goal, for the next few sessions that's what we pursue. Rinse and repeat.
But I still have an overall plot, overall goals the campaign is based on. It's just that those thoughts on where the campaign is headed can change based on character impact on the world and events or what the players are interested and excited about. Those small story arcs end up being quite linear even if I will react and adjust on the fly if and when the group meanders off into left field.