I know that 7 of the 8 demon kings are already confirmed, at the least.
If Rin goes there I think it'll be interesting enough, I know that me and my Nee-san do alot of roleplaying and we already have this huge idea that its gonna be like 8 small kingdoms surrounding a much larger one, then each kingdom corresponds with its king (Like Egyns is basically submerged in water, Iblises is like the fiery pits of he** you always hear about, Amaimons is a rocky and grassy area, etc.), then the middle is some over-exaggerated kingdom that Satan resides in.
Meh, whatever it looks like, it should be cool