I didn't downvote you, but I wrote enough games to hope no young programmer takes your advice.

Games, as iteratively developed creative products, always interweave engine with content. Trying to write a generic engine in parallel to your game is a surefire way to never release a game.

Even if you set out in advance to write an engine, you should do like most market leading engines and develope a game first, then extract the engine out from it (Quake, Half Life, Unreal, Far Cry... the only counterexample I can think of is Unity, wait no wiki says "Over the Edge released its first game, GooBall, in 2005.[4] The game failed commercially, but the three founders saw value in the game development tools that they had created to simplify game development, and so they shifted the company's focus to create an engine for other developers.")