No, it's not at all. Mechanics are not plagiarism. Plagiarism is taking the representation of another person's language, thoughts, ideas, or expressions as one's own original work. You are lifting their work as is and presenting it as your own. If I take inspiration from themes or parts of your work and reshape it into something new, that is not the same. Else we wouldn't have Elves and Dwarves running around in our games, nor any elements of what have gone before, even mechanically.
Would you consider Shakespeare to have plagiarized A Midsummer Night's Dream? After all, most of the characters in that play are not 'his'. He didn't create them.