While Latin-Greek mixed words can ond do happen, the clear precedent is the term "heptarchy" with an h.
That said, let's look at the dioceses of the Roman Empire around 400:
Rome, Constantiople, Antioch and Alexandria are diocese capitals, and Jersusalem (as patriarchal see) actually was added a bit later.
Ephesus has a good claim, as has Salonica.
Carthage is an obvious addition.
For Spain, we have an interesting situation: Carthago Nova (Cartagena) is old and the capital of Hispania Carthagiensis. More relevant might be that it is the capital of the ERE
Provincia Spaniae. But the same time, the Visigothic kingdom started to treat Toledo as the primacy see of (their part of) Iberia, for exactly the reason that it was not ion Constantinople's hands. I guess if the Heptarchical construction has to have legs, then a ERE-backed system with sees in Cartago and Cartageno will be more resilient.
For Gaul, Burdigala/Bordeaux and Trier are too far out, IMO. Arelate/Arles is not only close to the Med, wikipedia says that the[FONT="] "city reached a peak of influence during the 4th and 5th centuries, when Roman Emperors frequently used it as their headquarters during military campaigns. In 395, it became the seat of the Praetorian Prefecture of the Gauls, governing the western part of the Western Empire: […]. At that time, the city were perhaps home to 75,000 - 100,000 people. It became a favorite city of Emperor Constantine I, who built baths there, substantial remains of which are still standing. His son, Constantine II, was born in Arles. Usurper Constantine III declared himself emperor in the West (407–411) and made Arles his capital in 408.Arles became renowned as a cultural and religious centre during the late Roman Empire. […]. It was also a key location for Roman Christianity and an important base for the Christianization of Gaul."
[/FONT]I would say that Arles would be a good secondary option for the see of a "Patriarch of the Gauls, Britons and Germans", with the "Patriarch for the West" in Cartago responsible for Africa and Spania.