(A)What will happen in the world in 100 years?

(B)No one can foretell what will happen to the world in 1000 years'time. 

没有人能预言1000年后世界上会发生什么.

Pessimists are such people who always expect bad things to happen in the world.  

悲观主义者是总认为世界上会发生坏事的人.

解析：in/to都可以，但是侧重点不同：填介词in表示地点范围，填to表示对象

(1)what will happen+表示“时间范围的介词短语in +名词”

What will happen ina hundred years?  

一百年以后将会出现什么情况 呢 ?

No one can foresee what will happen in the future.  

谁也无法预见到将来会发生什么事.

The world is in constant change . No one knows what will happen in the next minute.  

请问这个美制单位怎么换算成英制单位.

(2)what will happen+表示“地点范围的介词短语in +名词”

Accidents will happen in the best regulated families.  

家规再严, 事端难免

Things like this often happen in busy streets.  

诸如此类的事故在繁忙的街道时有发生.

(3)what will happen +表示“对象的状语to sb”

If we had been spotted at that point, I don't know what would have happened to us.

如果当时我们被发现了，我不知道会有什么后果。

It's the best thing that ever happened to me.  

这是我所遇到的最好的事情了。