I'd love to see the version of the Believer theme that allowed it.
Honestly I'm just questioning the worship one god approach that most fantasy gods take when Pantheism seems much more appropriate for a world with so many gods.
Pantheism is the worship of all gods. But consider, the Middle East for eons was rife with multiple gods, and groups of people often favored one god, or a small set of gods, over all others. The notion of all/most gods is more a Greco-Roman view that is typical of the Decadent/Imperial cultures (RPG, Chap 13). Or consider, among Norse people, you could ask, "Who is your favorite god?", which doesn't deny the others.
Presently, characters are free to *honor* various deities, particularly those that are of similar alignment to their own deity, who is a *favorite* and the one who grands boons. Mechanically, this approach is balanced, though yes, it draws a line between honored and favorite. It also sets up a particular relationship where a character is a messenger, representative or avenger of a particular deity, for whatever reason. And it fits with a non-medieaval notion, in early industrial societies, of specialization. Just as townies are specialists, so too are deities and their followers. Even in a Decadent/Imperial culture, we might think of one's "patron deity" (to use the old AD&D term) as one's... divine patron. This doesn't exclude other gods. It merely frames a relationship with one particular god as special. Anyone, now I'm perhaps sounding like I'm asking how many angels can dance on the head of a pin!
All this said, I entirely agree with you that there needs to be space for pantheism/polytheism because many campaigns will have a Greek/Roman worldview, or whatnot, where PCs petition whatever god appropriately fits the context. I'll make it so!