Let's Find a theme for a game, any thought?!

The theme of the game is like the first date with a girl on the web, you don't know her yet, but by looking at how she dress, or look like, you start to judge her. (seems a bit unfair, but this is how things work) After that you play the game, you may find:

1. How she dress is suitable to the personality

2. This is so wrong.

In situation 1. this is what we all like to find, a good game with a good mechanism! (or the other way, a meh game with a meh theme)

Rattus- A theme that is looked like is based on novel or some book, however, 

it's backed up with history and even it's expansion does that.

In 2, I will interpret as "a good game with a boring theme or a good theme with a boring game".

Sometimes, I think the artwork and the theme does upgrade a rather usual game to the next level. From the business point of view this is great, but for experienced gamers, it probably doesn't work either, a boring game is still a boring game. 

River Dragon - The dragon is not really plays a strong role, not a strong theme game but a very colorful background and components making the game different.

Viva Java - Coffee!? Yes! from my point of view, a good theme for a semi-cooperative game, coffee beans, research skills, overall, I think the theme is as great as the game itself.

In the case of my own game, the mechanism and the theme of the game nearly made up at the same pace, I think this is what usually people do but the difference is, I usually wouldn't agree in changing the theme of a game.  

One of my colleague usually came up with a mechanism but without a theme, i used to help him to solve this problem, but after a few times, I think it's time for him to grow, therefore, rather then provide a full theme, I try to inspire him with some hint. I am still trying this, as this might give a even a better theme then the one is already in my mind. 

By comparing 2 of my games Office 21 and Gold Rush, GR is a way better concept then Office 21.

One of my friend think the theme is just okay, I can't fault him, but I think it's a suitable theme probably the game is made for the theme. I am confident in the game which I think it gives a twist on the usual card drafting game, and I am planning to play the theme a bit more since I have a bit more time till the release date! 

I would love to know, how does the theme comes up from many games and one more question to think? any theme that has not yet been made into a board/card game?