I think the T13 boss will be the Alaphanaud twins, we will come down to finally being able to cut off Bahamut from ever returning but this will also mean the end of their grandfather. It's almost obvious if you think about it. They are obsessed with Luisua (grandfather and old man who binds Bahamut) The only reason Alaphaud seems to be following you through the coil is to find information on his grandfather, he freaks out when ever he gets a taste. The sister is already out on some pilgrimage trying to find the Grandfather, maybe returning corrupt and more powerful, with a dragon or something similar perhaps. Alaphaud also carries a mean looking weapon on his back that we never see extend, this could be anything, especially worthy enough for the final fight in the Binding Coil. Kill the Twins, epic cut scene of finishing Bahamut, some tease on how he might not be actually dead. Very sorry for butchering the names, that is how I say them in my head, no idea how they are actually spelt.