你问的句子，Ghost was to be a stealth-action game. 是表示将来的“be+动词不定式”结构，不是“系表结构”。

关于这个问题，一般语法书都有论述，大多不够详细。下面详细解说：

在不同的语境中，“be+动词不定式”结构可以表示多种不同的语义。

一、表示“计划”或“安排”。

This line is to be opened to traffic next week. 该线路将在下周通车。

They are to meet at the gate of the school. 他们将在学校门口见面。

二、表示“命令”。一般译为“必须”或“不准”。

No one is to leave the room without permission.

This naughty boy is to stay here until class is over.

三、表示“可能性”。

Are you to pay the debt for your wife?

The lovely puppy is nowhere to be found.

四、表示“意图”或“打算”。一般译为“想要”。

If you are to be there on time, you had better hurry.

If all of us are to remain friendly, we must be sincere with one another.

五、表示“应该”。

You have done so well. You are to be rewarded.

Such nasty questions are to be avoided.

六、表示“不可避免”或“注定发生”，意为“必定”。例如：

Your plan is to be a failure.

The murderer was sentenced to life imprisonment and was never to see his family again.

She did not think too much of her unkind acts. However, they were to be harmful to herself, too.

Tom wasn't worried about his head injury, but it was to be troublesome for him later.

七、表示“期待”。

The actual use of the newly invented equipment is still to be determined.

Henry's appointment with the firm is yet to be confirmed.

八、表示与将来不符或相反的“假设”。

If it were to rain tomorrow, we would not go outing.

Even if you were to take a taxi now, you would not be able to arrive at your destination on time.

关于你提问的句子：Ghost was to be a stealth-action game. 应该为第六种用法，“将来必定是；将来注定是”。

如果直接说：Ghost was a stealth-action game. 意思就变了，说明当时已经是。