The first parts of this world were written in the 1980s, when first edition AD&D reigned and Legends and Lore was the 'official' source of religions. So my world started off with a Celtic vibe. Later parts of the world were written for Al Qadim, D&D 3.0 and then pathfinder, and the deities followed that progression. so the faiths of this world now follow that pattern. The Old Lords represent the original world, the 3rd Ed deities spread from the west, when they met up with some Al Qadim deities from the south (now represented by Sakha), and finally the Pathfinder deities came in from the east, and now dominate the region. Although Pathfinder deities predominate, you will find the flavour of deities from those rules sets, spread across my world.
Perhaps the major difference is that most people are pantheists - and venerate all the deities within their local pantheon. They might favour one above the rest - but they are liable to say a prayer to any of the deities of the pantheon, when the time is right.
Before you look into deities, you should be aware that some faiths only grant up to 3rd level spells, and Quasi-Deities can only grant up to 5th level spells.