I find that it is a good idea to establish early on if the RP is going to be a sandbox or have a defined finish point. Sandboxes can be easier to run just because its easier to throw characters in and out, but at times feels a bit empty as plots are harder to evolve and it can feel like everyone is floating in anti-gravity.

Meanwhile having an defined end helps people along the path, even if at times it can feel somewhat linear. You just have to remember to stick to the path, otherwise you end up going off it and end up never returning to it.

I might have to write a GM guide on this... I have the urge to write one.