On the topic of Arianism I had a pretty big idea to flesh them out.
Historically while the Germanic people were christian they still refused to give up most of their cultural tendencies for most of the migration period. Only after the Visigoths began to settle down into Hispania did they really start to resemble a european state and became a settled people.
Considering this mod still takes place in the migration period I would like to see the germanic tendencies of the Goths simulated by making Arianism a pagan religion that reforms into a Nicene heresy/Christian Heresy after the Great Schism. The unreformed religion would be in the pagan group and allow raiding, river movement, concubines and the ability to intermarry with christian faiths while the reformed Arian heresy could perhaps only allow river movement but still is in the christian group.
I imagine the holy sites of the pagan Arianism could be centered on Hispania to simulate how the Visigoths managed to survive and prosper as a nation. So for example they could be in Galicia, Sardinia, Rome and Roussillon.