Well that Bahamut is to huge, plus it got no animation if they were to add Bahamut as an avatar they will make a new model I'm sure of it.
On the other hand Diablos looks most promesing, THIS is intresting if he got all the animations already plus the size is more what I like then this is something I want!
I mean it's obvious they have to add more summons anyway, as they have to add more abilities, spells and items to other jobs if they want us to continue playing this game for long...
So I guess it's all about waiting, some people say the new HNM's are screaming raise of level cap but i'm still sceptical because of things like Refresh.
Anyway post your opinion, I like to speculate in stuffs like this.
EDIT: After a closer look Diablos is still to huge
