just wanting to check if this is allowed before I upload some projects

There is lots of "fanart" games on itch. Is there something special about written works?

I believe that stuff usually falls under the parody and transformative clauses of fair use. Money is not the deciding factor, but all fair use cases are individually decided with those 4 points of that doctrine. Fanfiction should be viewed favorably in at least two of them. It creates new art and of course imho, in most cases the copyright holder just cannot claim that the fanwork hampers his chances of exploiting his works. Quite to the contrary. People do not avoid seeing your movie in cinema because they read some fanfiction about it. You are (usually) not competing against the copyright holder, but helping the original work to become/stay famous.

The internet tells me, that fanfiction in particular seems to not be legally challenged all that much. The wikipedia even claims that there just are no law suits known in regards to fair use. To speculate, maybe because lawyers laugh at clients trying to take down fanworks, as those would be so obviously not competing and harming the original ip. It is even in the name, fanwork/fanart/fanfiction. No one could mistake those as coming from the original owner, nor is it competing, and it is creating new things.

All that being said, owners of the works that got fan-ficted, might still complain.