If it is a sandbox then I think something slanted to a skill based leveling system is more appropriate, IMO.
Why can there not be both Class and Skill based in one game?
This is just an idea, feel free to improve upon it, but why not? At the base all classes use skills, why not have the foundation of the game be based on a skill system. Then have an additional layer of classes on top.
I hate class systems because you are stuck as that class and (for me) kills creativity.
Lets say you create a character, you can pick a class which automatically selects skills for you to use or a custom option that allows you to pick and choose skills to work on ala UO. If a class is chosen it places you on a rail so to speak, a linear path (just like other MMOs with a class system) and may (or not) automatically increase certain stats. Now later you can derail yourself from a class and increase/decrease certain stats if you so choose.
I like the option of gaining different skills (thus dropping others) later in a game if I can.
I would be highly disappointed if Pathfinder Online jumped on the ole' class based system bandwagon.
"Z"