I don't believe it is possible for atheism to ever be anything more than critical theory because there is no affirmative case for atheism. The only argument of atheism is to argue against religion and to criticize religion.
So my question is... Is it possible for atheism to ever be anything more than critical theory?
Atheism isn’t a “critical” theory, whatever it is you mean by that; only because atheism isn’t a theory. You’re right in saying that there is no affirmative case for atheism.
Atheism is a rejection of the affirmative case for a god or gods. It’s the null hypothesis. Atheism is just scepticism of the claims made for a god or gods. It’s just being unconvinced. It’s simply not having a belief.
That’s it - nothing more.
It isn’t materialism, though many atheists do subscribe to materialism. It isn’t philosophical naturalism, though many atheists do subscribe to that as well. It isn’t what believers call “scientism”, though some atheists do think the scientific method is the best way to ascertain truth - lower case t.
If you want to know what an atheist believes or doesn’t, beyond whether or not a god or gods exist, you have to ask them.
Some are communists while others wholeheartedly believe in laissez-faire capitalism. Some believe in ghosts, psychic powers, chakras, and telepathy. Some think anything explained using the supernatural as the explanation is baseless superstition and lazy thinking. Some hate LGBTQ people and are racists, others want to promote tolerance. Some are hardcore traditional conservatives, others are freewheeling liberals. Some want an authoritarian government while others are anarchists. Some want to rid the world of religion while others respect and admire religion.
To try to categorize atheists and atheism beyond the god question is to make a bold but foolhardy assumption. I suggest you don’t as you will probably be wrong in that assumption.