Dunno why I never posted on this thread, but here goes.
First, my apologies to The Imp as the OP. Long time, no see.
Second, the prophecy of the valonqar is to beware the little brother. Valonqar is Valyrian for younger brother, but we learn from Maester Aemon that the word for prince (in Valyrian) is actually gender neutral and is the same word for princess... thus we can assume that mayhaps valonqar is gender neutral as well. This can change the prophecy of the valonqar to mean younger sibling and include sisters as well as brothers... and this must be done in order to make Arya available to be the valonqar.
Once that assumption has been made and Arya is available to be the valonqar, we realize that all younger siblings in the entire world are now available. I don't know much about Medieval European birth rates nor the mortality rates of children, but I assume (and I think GRRM does as well judging by his family trees) that even if parents only had one child live to adulthood, they had numerous children along the way. I think allowing valonqar to univerally apply to all people who were not the very firstborn child means that we can safely assume that the great majority of people in GRRM's world could be suspected as being the valonqar. In my opinion, the purpose of a prophecy is to narrow down the possibilities and not to increase them to over half of the world's population. Thus the valonqar must be strictly applied to Cersei's own family, i.e. Jaime and Tyrion (or even Jon and Dany for you nutters out there.... like me) or taken to the next generation and applied to Myrcella or Tommen (though in this case I think the prophecy would have been to beware the offspring and probably have been directly tied back to Cersei's question about her children). The only exception could be for a specifically titled person such as Brown Ben Plumm the commander of the Second Sons mercenary company. As the leader, he might be called THE Second Son. Ben was recently hired by Tyrion to help him reclaim Casterly Rock and destroy Cersei. This could allow GRRM to fulfill the prophecy in two ways... Tyrion is a younger brother and Ben is THE Second Son... both could be seen as valonqars.
This could be a nod to Tolkien, GRRM's favorite author growing up. In The Lord of the Rings, the Witch-king claims a prophecy that "no living man" could best him in battle. Tolkien used Eowyn (a female) and Merry (a hobbit)... neither one a man (one by gender and one by race)... to defeat the Witch-king and thus doubly fulfill the prophecy.
