There is no real religious system ingame YET, but the piety is there. In my opinion piety is more like a christian attribute not like viking or shamanism. And this leads to my religious suggestion, we should have three, kind of world religions like the three chooseable races.
They appear to me as christian, viking and some kind of shamanism like in the past in mongolia. But we shouldn´t choose this already existing religions, no we should build similar religions. This could be done by the Loremasters or interested players.
Every race would have their own religion and goods they are worshiping. Every race would have their own grand religious projects like building a cathedral, build a temple or create a holy grove. This building could be a center for religious rp and pilgrimage. Then we would have three grand titles for the religious leaders (those who reach piety 100 first), pope, grand shaman and high priest. Each one could reach 100 individually and then starts the fun part of the race. Now they can gather their warriors, crusaders and fanatics which will kill each other until one leader / religion stands out. This religious leader will now take the part whis was for the pope in the original idea, who could change taxes, and mess with some environmental things only this player could do. (If you have some suggestions with more detail, feel free to add them as replies!) After one year IRL the bannerman will call in the armies again to choose a new leader. If the player looses intereset in the title of religios leader either the next one with 99,99 piety will get the title or the battle will be done earlier to give the other religions a second chance. I know many of you are uncomfortable if it comes to religious war, but this was part of the feudal life. And so it should be here in life is feudal, remember it´s just a game. Another funny idea would be that you could worship the gods of the NPC wildlings, and try to let the old gods rise!
Pls feel free to add more suggestions of this kind as a reply and maybe we can work out something that the devs would like to add!