I'd guess that without the invention of montheism the commonest system would be henotheism, ie singling out one god from many for special worship while fully acknowledging the existence of other people's gods.
In the modern world it would be like everyone accepting there are (at least) three gods, yhwh, god and allah, all of whom are venerated but (depenun on your tradition) one is especially so. With montheism comes the idea that only one god is valid and it is wrong to worship anything else. A polytheistic put look makes it much easier to accomdate the gods of anybody else.
So the old, pagan religions would have carried on. Being of north european heritage, I'd probably worship the norse gods mainly, but the mythology and doctrine could well hae got mixed with up the gods of the celts and the greeks by now and I'd believe in all of them!