Game Clones

If you're selling content that belongs to a certain company, no it's not legal. For example, if I made a game that had similar game play as Zelda but my own content it wouldn't be illegal. Tetris on the other hand as been repeated several times and sold under many names, however I cannot answer your question for if Tetris is legal or not, but in general making your own game is a much better option then clones. Don't fall into the trap that so many others have by making clones of games that contain copyrighted images, even if it's not going to be sold.

All assets such as artwork are made by myself so the game itself would not contain content from any other title.

The general gameplay itself is not covered under IP laws. That is generally safe to copy.

Any similarity in names is covered under trademark law. For the Tetris example, several games with "tris" have lost court battles. The same is true for variations on Mario and other well-known marks.

Similarity of images is covered under various laws. Even if you made it yourself, it the image resembles or could be confused with theirs, you are opening yourself up for a lawsuit.

Similarity of gameplay can potentially cause an issue. If your gameplay is extremely close to the game you are cloning, the IP owner could sue you for several different types of IP infringements.

IP law is not limited to trademarks, patents, and copyrights. It includes many other topics like design rights, trade dress, database rights, author rights, "related" or "neighboring" rights, trade secrets and "know-how", and so on. The closer you come to copying their work or methods, the greater risk you run of infringing on their rights.

You don't need to infringe on their rights to face a legal challenge. Using any clone or any similarity has some legal risk. Most people who post here cannot afford a legal challenge, and as such want to minimize the risk. The least risk is to create something completely original, but there is still a tiny risk of lawsuit.