It's funny. You would think most people would have a full playable game in their concept with a goal. And a challenge.
But, no, these are often missing.
And "Fun", "Good visuals", "Theme" or "Good gameplay" don't create a unique game.
Here's a recipe that works often:
1) Something new + something old
Have something new and something old in the game idea. See here more details, the Tetris + Scrabble is a sample of this.
2) Include theme + mechanics + variation
We don't really work much with our idea to see how far it extends or bends. I recommend this highly. There is numerous part and exercises here to improve on that. You can utilise the theme from here and learn the core of your game from here.
3) Insert Wow and unique factor!
Your idea should have something mind-blowing in it :D You can get help on that here!