The theory is usually that he's one of the major gods, not a demigod.

I feel like there was a Maia who mated with an elf, but I don't remember with any certainty. Demigods aren't a major feature of the world.

EDIT: https://lotr.fandom.com/wiki/Melian is the Maia who mated with an elf. Their daughter, Luthien, is a normal elf, in much the same way that Elrond is a normal elf while Elros is a normal Numenorean. (Though Luthien doesn't seem to have been given a choice.)

It's probably too blatant for Tolkien to have cone up with but Tom Bombadil has the same number of letters in first/second names as Eru Iluvatar. The ring has absolutely zero effect on him, but that would probably be true of any of the Valar just as much as Eru himself.

If this was built as if it was MCU they would have had a small cameo then a full movie on Tom Bombadil.

I’ve been expecting a 15-20 shirt put out that would fit into Fellowship with just Tom Bombadil and Elijah Wood et al. The cast and effects should be minimal so would be a great student project that would probably attract the few actors needed to make it.