I'll add that from Itch.io's point of view, we're not going to check beforehand, unless you're selling your game for money, and then only to make sure the game itself doesn't belong to someone else. We don't normally look at the assets used.
The moment someone complains, however, we're going to look into it. That's mostly because Itch is hosted in the US, and US law holds web services like this responsible if they don't take active measures once they're notified of a problem.
Fortoj is right. Make fan games if you love a franchise that much. You might get lucky, especially if you make no money from it. But it's best to use your own original characters, stories and so on. Buy art and music if you can't make your own, or find some that's under a Creative Commons license. Own your work, it's a lot safer that way, and you can do a lot more.