I have plans to introduce a monotheistic religion into my new gameworld. I allways thougth that haveing a monotheistic religion would make it easier to me to build a believable world. (More similar to RL medieval times.) Other religions (and gods) would exist, but the main religion would be this monotheistic one.
It would have saints (or saint-like beings) who were originaly living persons, but did some great deeds and so became minions to the god. They could fill the roles that different gods play in polytheistic religions (for example granting different domains.) Most people would simply venerate (is this the right word?) the god, but some would dedicate themselves to these special beings directly (similar to monastic orders IRL), and even normal people could call upon them in special cases.
I think this will give me a good mix.
Or you could use the version introduced by George R R Martin in his Song of Ice and Fire series. There are seven gods (Father, Mother, Maid, Crone, Warrior, Smith, Stranger), but they are allways venerated together. No seperate churches, no real seperate priests. Some people (and priests) even think that they are only seven aspects of the same god.