The whole idea is using Iskandar inherent qualities (supposed to be related to Zeus, deified as a part of the Olympians in some traditions, reincarnation of Amon) to turn him into a Divine Spirit.
The problem is that you need a connection to the Iskandar in the Throne (Faker, by using a death and rebirth initiation rite inside the labyrinth to Saint Graph Ascend her into Iskandar), a lot of time due to Divine Spirits needing godly qualities and faith to be spread among the people (that's handled with the Emiya's time compression magecraft that Heartless got hold of) and a lot of energy in a proper place (provided by the Grand Roll itself as during the ceremony the dragon's heart of the Spirit Tomb Albion is opened, making it the best place to get a mystery wave big enough connected to the inner sea of the planet to turn a Heroic Spirit into a Divine Spirit).