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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.

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.

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.)

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.)

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.

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.