你应该区分to do不定式和疑问词+to do不定式。这是二种不同的结构。

to do不定式可以做除了谓语之外的所有句子成分，而疑问词+to do不定式却只能做名词性的成分。因为疑问词+to do不定式本质上是名词性疑问分句的简略形式。例如：

I don't know what I should say. = I don't know what to say.

He is not sure when to start. = He is not sure when he should start.

名词性疑问分句做介词宾语的时候，有时介词是可以省略的。例如：

I have no idea (of) what I can talk about.

You haven't answer the question (of) what you will do next.

以上二句中的what从句都可简略为疑问词+不定式结构。介词of用或不用都可。

I have no idea (of) what to talk about.

You haven't answer the question (of) what to do next.

所以，你的问题与 普通to do不定式无关，而仅仅涉及疑问词+不定式这种特殊结构。不能夸大为to do不定式结构。