I am, as usual and last I checked, not a lawyer. What follows isn't legal advice from a legal professional, it's layman knowledge gleaned over a couple decades of engagement with the practical impacts and relevance of intellectual property on citizens of North America as publishers and consumers of media. If you're concerned that your actions might provoke a Cease & Desist letter or need to be defended in court, engage a lawyer for advice before proceeding with those actions.
This answer is also tuned to the fact that you are in the United States.
General ideas can't be copyrighted, so borrowing ideas is not going to cause trouble. Borrowing specific named items can even be fine—the original D&D borrowed mithral/mithril wholesale from Tolkien's novels about Middle-earth, including the name and function, since the name is not a trademark and the idea is not something the Tolkien Estate can own and prevent others from using. (The situation is different when taking story elements though, such as places and characters: those are usually considered impossible to divorce from their original source, making them subject to copyright protection. The difference is subtle but an established precedent legally.)
When renaming things, it gets even farther into the clear: if you like the idea of Cormyran duskmantle leather as a crafting material that you read about in a game supplement, borrowing its (non-mechanical) use and function and calling it swiftleather is not going to be a problem.
Even things that are recognisable aren't issues: you can totally have a character type option that loudly emulates Drizzt Do'Urden without using that name, and everyone will get the reference—but the idea of a wilderness warrior form an ostracised race fighting with two curved swords is not copyrightable. (Of course, borrowing the character itself is a problem: if you have a pregen of that character type option named Trizzd who is a dusk elf who left Marzipanbarrens and forged a friendship with the Woman of Shinymoon, then you're on thin ice even if you're going to argue it's satire; if you have a pregen named Drizzt who comes from Menzoberranzan and is friends with Bruenor Battlehammer then that's copying the character itself and a no-go.)
For a practical example, check out Epyllion: it's basically My Little Pony except with dragons. Nowhere does it copy from MLP, but the inspiration is clear to anyone who is familiar with the show. It's even set in Dragonia and the major theme of the game is the power of friendship. As ideas go, MLP is a very clear inspiration, but it doesn't rip off the places, characters, and names of MLP—instead it took the themes and structures of MLP, drew from other ideas (original and from other inspirations) and made a new thing to express the same kinds of ideas.
In general, ideas aren't owned by anyone, just the actual work that people put those ideas to. If person A includes a red metal that floats in water in their game, the owners of Glorantha don't have any grounds to go after them for it, even if they can prove that person A has read about Gloranthan metals. What people are given temporary monopoly ownership over is what they actual create around the ideas: the actual media they produce based on the idea.
If it were otherwise, the entire project of having a culture would be impossible. Imagine if there could only be one cop show, one superhero movie, one book about alien contact, one painting of sunflowers. The world would be a very different place if copyright covered ideas, not just the words and pictures used to express ideas.
So yes, you can take inspiration for things from all over. You can have character classes that are homages to character ideas from elsewhere as long as you're not re-using their work, only referencing their ideas. You can have materials that are references to other games, so long as you're not just collecting together everything unchanged. Make it an original composition, not just copying, with an original expression and value, and you'll be doing what everyone else has been doing since the second RPG was invented after being inspired by the first.
