Everyone knows monotheism and atheism aren’t the only options. For example, you could instead be a pantheist, or a polytheist. But it doesn’t stop there. There’s a number of more exotic subvarieties you’ve probably never heard of:
What counts as a deity? Sciotheism means you believe ghosts are gods. Zootheism means you believe animals are. Uranotheism means you believe celestial bodies are divine. Myriotheism means you believe in an countless number of gods. Physitheism means you believe aspects of nature count as deities. All of these have been held to be true at some point in time. Nowadays most people probably hold to psychotheism, the belief that divinity is spiritual not physical.
What deities do you worship? Henotheism means there’s more than one deity, but you only worship one. Kathenotheism is when there’s more than one deity, and either you take turns worshiping different ones, or you worship a different one depending on where you are. Monolatrism is when there’s more than one deity, but you only worship one, and think everyone should do the same. Some people think the monotheism of Abrahamic religions started out as monolatrism.
What qualities must a deity possess? Nomotheism means deities are bound by universal laws just as we are. Dystheism means the divine realm contains evil. This can be a polytheistic belief that one of the deities is evil, or it can be maltheism, the belief that God is evil. Note that believing a deity to be evil does not mean you do not worship it. You might worship an evil deity for pragmatic reasons, or because you think there should be a balance between good and evil, or because you yourself are evil.
If there’s only one deity, who or what is it? Hylotheism means that God and the material universe are identical. Panentheism means the universe is contained in God, but the universe is not itself God. Some people find this a distinction without a difference, some think the difference between these two of the utmost importance. I’ve seen it claimed that most modern Christian theologians are actually panentheists. Inherent in this position, as I understand it, is the view that God did not exist before the world existed. Autotheism , the belief that God is self-subsistent, that is, not dependent on any other being for his existence, seems to me opposed to panentheism in spirit, but I don’t think the two are logically contradictory. Autotheism  is the idea that you are at least potentially divine. This can be an extreme subjectivist belief that you’re the only deity, or it can be the gnostic view that you contain a divine spark and can undergo apotheosis, or it can be the Christian view that you can undergo theosis/divinization, that is, can be lifted up by God and allowed to participate in his divine life.
This overabundance of -theism words amuses me. I’m not sure why. Perhaps because they pack so complex a thought into so little space. They’re words for conjuring up memories of philosophical arguments.