Disclaimer: This thread is created under the laws of assumption. I am assuming Odin/Dark Rider is an avatar and will be added to Summoner's spell book in the future, seeing how in other Final Fantasys he was a summonable character. (I know, I know. So was Bahamut, you can't fall back on this every time an avatar thread rolls about, nor Phoenix.)
Ok, for one, SE did say something about new avatars for summoners, we're left with Odin and Alexander, who are the Bahamut/Phoenix of ToA. COP gave us Diabolos, Phoenix is sorta...well, in a sword/dead, and Bahamut doesn't seem to be planned as a summonable judging from his strength, and end game boss status. Now we have Odin and Alexander. Alexander is currently, damaged in TOA story line. The under sea ruins is his body, and his 'legs' are split around Aht Urhgan region. So at the moment he's damaged, and it is said if he revives he and Odin shall clash, and some shit will go down.
Now, moving away from Alexander, we have Dark Rider. From recent Cut Scenes he's off his horse, and Luzaf, is (possibly), next in line to be a dark rider. This being said it would seem certain that in the future we'll likely fight Luzaf as Odin in a BC, like Diabolos, and SE can easily make another BC where you can obtain him as an Avatar. But this isn't what made me speculate about him.
Look at Odin's stance.
http://i5.photobucket.com/albums/y15...RD/OdinATK.png
From glance, I see a semi reminder of Ifrit when he's summoned. Also during the CS he appears to 'float' like Ifrit does, hunched over.
http://i5.photobucket.com/albums/y15...OdinSummon.png
This picture gives me the image of not only meteor strike, but a bloodpact stance. Not a bad Dark based elemental attack (assuming it was Dark).
Naturally we have no idea what moves he will have, and this is likely because:
A.) Dark Rider's hard to find
B.) Noone lasted long against him.
C.) There hasn't been an oppurtunity to throw more than 6 people (est.) at him. {As only Partys who stumbled upon him, engaged knowing they'd die.}