I’m using a ton of 5e material since it was published to the Creative Commons. All you have to do is include the following attribution in it and you can use it freely:

“This work includes material taken from the System Reference Document 5.1 (“SRD 5.1”) by Wizards of the Coast LLC and available at https://dnd.wizards.com/resources/systems-reference-document. The SRD 5.1 is licensed under the Creative Commons Attribution 4.0 International License available at https://creativecommons.org/licenses/by/4.0/legalcode.”

I’ve been playing 5e for many years and it’s a system many people are already familiar with. Of course, as with most DMs I’ve played with, my setting and rules are heavily homebrewed. It’s an Early Modern magipunk. The story is about a group of outlaws on the run. It’s been fun to write.