the harder you work, the more questions you will think of _____

to ask这句话是这样的
You will think of more questions to sak.

I have a question to ask.

I have something to say.
都是这种语法结构的。

the more----the more--是一个前置的结构。所以原题中还原成正常句序就是you will think of more questions,也就是说of已经有了自己的宾语，而不是接doing。

因为此处所缺少的成分不是of的宾语。