What are the legal issues of using game play, and game dynamics from another game. For example, I would like to make a game similar to Mabinogi owned by Nexon. It would have my own name, and my own story. It would contain my own world. But the way the skills and game play work are similar. What is plagiarism here? I know final fantasy game play and dynamics have been copied for years.

I am not a lawyer, but I suppose if you use your own assets and world, it's not a problem.

Just look at this list https://osgameclones.com/ which barely scratches the surface of clones/remakes of old games. Hell, most of my projects are clones of existing games - my space project is inspired by Stellar Frontier, my racer project is inspired by Need For Speed games (Underground 2, Most Wanted, Carbon), and I have a new project in the works which is inspired by stealth games such as Deus Ex 1 and Rainbow Cell Rogue Spear or Hitman series...

As long as you put your own assets, world and do not copy 1:1, you're probably fine. For instance, my racer has no named/brand cars, just lowpoly squareish bodies... my space game has fairly extensive procedural generation system and the planets actually orbit, which they didn't in the original probably due to performance reasons (it was 1998, after all)... and my new project will probably use procedural generation, too.

I did read about this on the wikipedia. In my opinion, it would be morally incorrect to patent or copyright game play element. That would be like a copyright on the idea of dancing. It would be different. They said something about the feel and response being copyright protected. Game mechanics can be copyrighted, something about them can be.

but on that, I can see someone making a copyright on the idea of gravity. Or even polygons