Oh I really haven't thought about this one.
Between us (me and DH) we have 5 siblings and all have already taken on the role with our first 3 children. Our kids all have 3 Godparents a piece, 1st child - my sis, my bil(her partner), my bff; 2nd child my sis ,bil (her DH) my other bff; 3rd child, my other sis, bil, sil.
So, Ive ran out of siblings lol -thought my number 3 was gonna be my last, so we squeezed the last three in together -oops, probably an oversight!!
DH's friend's havent' been asked before, maybe they are mature enough now lol -Needs some more thought though I think.