Naaaah 8 should be next, best final boss themeSSS in the series, one of the most epic end fights in the entire saga and one of the best final dungeons with a villain that could very well work.
Ultemica phase 1 - Premonition
Ultimecia phase 2 (Griever Primal is summoned) - The Legendary Beast
Ultimecia phase 3 fight - The Extreme
Ultimecia's Castle - The Castle
A 3 part Ultimecia/Griever fight trying to prevent her from compressing all of time and space so that she can become a god. I can easily see her manipulating and controlling characters in the story to advance her goals much like the Ascians
Also, Necron who is death itself:
I can easily see Nero calling out to NecronQuote:
In Final Fantasy 20th Anniversary Ultimania - File 1: Character Book, Necron is described as, "a being awakened by Kuja's fear, despair, and hatred, which called out to it as he learned of his mortality, just as his ambitions were within reach".
