I think composition of how it's formed can certainly be looked at but 4 of the 5 biggest military powers in the world (when you factor in capability, hardware etc. rather than just raw numbers) are represented by the permanent members with France at 6. I certainly think they need some form of a seat at the table even if it's no longer a permanent status. Just for the simple factor that any large conflict if likely to involve one or more of them.
15 elected members? Probably not the best idea.
15 elected members? Probably not the best idea.