First I have to say that I love this thread and all of the speculation in it.
I don't think that Louisoix is Thallak or Rhalgr, I think he's a summoner, just based on the fact that his staff has a horn on top. I would lean closer to Thallak than Rhalgr, though, looking at the pictures TenraiNagi posted.
I think that he is going to end up sacrificing himself in some way to save us/Eorzea.
After you defeat Van Darnus, Cid says something along the lines of "we won't have to use the 'last resort' that Louisoix keeps muttering about." But, we know already that it will probably come to a last resort scenario, especially from what the trailer tells us.
In the trailer for ARR, it transitions straight from his face into a roaring Behemoth. So, maybe he will end up sacrificing his body and end up transforming into a behemoth as a result of all that aether bending/warping all that space and time and matter? Just a thought. It does seem to have a beard in the trailer. I think it would be cool. :P
Also, aren't we forgetting that in the lore, Dalamud is supposed to be Memphina's loyal hound? Maybe it houses Fenrir.
From Wikipedia, "due to the gods' knowledge of prophecies foretelling great trouble from Fenrir and his rapid growth, the gods bound him, and as a result Fenrir bit off the right hand of the god Týr." He is also supposed to kill Odin. Though, I think it will be full of all sorts of bad things, like mandragoras.