I think the problem with it being anyone other than Charles, William etc, is that it risks turning the position into a political position and therefore would lack the ability to unify an organisation of very, very diverse nations.
For example, if it were a rotating position between heads of government, there is no way Margaret Thatcher could have been acceptable to African and Caribbean nations given her position on sanctions against South Africa. Likewise some heads of state/governmanet may be considered to have unacceptable human rights records.
We only have to see from the choice of Olympic and football world cup venues how non-political decisions can become geo-political rows or open to bribery. What should be a simple decision based on who is best for the job, has the potential to become divisive.
I can understand the argument that Charles will only be monarch of a limited number of countries in the commonwealth (and William and George probably even fewer) and therefore why should he be head of the commonwealth. There is too, the uncomfortable reminder of Britain's colonial past. I am also generally predisposed against inherited titles and positions but, in this case, I genuinely believe the King or Queen is best placed to be the unifying force. Charles has spent his life visiting commonwealth countries and William and George will do the same over their lives. Who else would have had the contact with all the commonwealth countries that they will.
