He definitely needs to be a boss fight.

While FFXIII-2 practiced this, it would be really awesome if they made the Void Coliseum and every major patch or expansion they add in a familiar character of the FF series as a boss. Gilgamesh would fit perfectly here.

Or can take the FFXII approach with him and have him involved in a side quest.

But overall, however they decide to put him in here, his appearance in this game is inevitable.