I tried something like this once and it seemed to start off quite well from what I can gather.

I think part of the trick is when the person makes their character sheet you need to include something like a motive or mission so that they can start the ball rolling. Some of the hardest aspects about sandbox RPs is that if there isn't much happening at all and the players can run out of ideas very easily. They do need to push themselves and create their own conflict, but they also need a reason for being there in the first place.

When you look at the character sheets you might be able to spot some of these because the player will either design a character that lives within that world, or someone that is simply placed within that world. A character that lives in the world will have a history and aspirations. They could have a desire for revenge, or a sense of justice that pushes them. A character that is placed within the world will almost be like a Mary Sue in a way, a self contained character sheet with some resolve already written into it and no real future to write about.

Eg: Lets say you have a character that lost their family and then trained to get revenge on the killers. You could say that they start their journey on the hunt for these murderers, but what about if they were to go back even further and start their journey just after they lost their family, so they're weak and they don't know who they are chasing. Instantly you have a lot more content to write about and you have a self contained story that can fir into many sandbox scenarios.

So if you're GMing a sandbox I think the best thing would be to make sure that each player will have enough that they can write about and if need be, tell them to wind back their clock and start their adventure earlier in order to give them more content.