I really like flyleaves for ringed organisers. They add extra pockets to very plain binders, and provide a place for overflow in binders that already have pockets.
I have two vintage Filofax flyleaves:
one flyleaf that came as part of the Grace Scurr edition Filofax
one from a Graf von Faber Castell Epsom binder
and several Van Der Spek flyleaves, which can be custom designed in any configuration you like. I tend to have secretarial pockets and zippers on mine
Gillio binders also come with plain (no pockets) flyleaves