Add Oil is a bad translation for a chinese "Sprichwort" which means you should give your best and go on working xD

I have to disagree with HeelX about the sentences. I like them and it makes it sound more attractive/interesting to me.

Great and detailed feedback!
Very true observations!
I'll think about it!
Thanks a lot!

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As explanation:
The reason why I work on a cover is not that it shall look like this in the shelfs, it is meant for a potential publisher to imagine which arguments this game could offer when it stands in the shelfs.
When it cames to publishing, most probably the publisher will employ a designer to make the cover on his own.
Nevertheless, this one should be good, too.

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More opinions about the sentence? It is meant to tell the contradiction/the suspense: they are selfish, but belong to each other/ need each other/ depend on each other.
A contradiction that shows the task: keeping together who doesn't care for each other!

...hold a competition

My opinion (and I'm no expert):
The title should show some artwork related to the game and not a captured ingame shot. The back of the box is for screenshots
If you don't know what to take as background, then use a single color or some non annoying pattern. I've seen this on several game boxes. The main character(s), the title, and some simple background. Other boxes show some painted scene of the game world.

I also agree that the title should be placed on top, this is where you usually look first.

You are probably right about the artwork instead of ingame shots at the frontpage.

At least, I'm healed again from my illusion that I can do this in a few hours without any extra effort.