I would say that we can dream about most things but there are a few obvious limits as there are types of thoughts that will make you wake up:
* Reading is difficult and typically books in dreams don't have words in them.
* "Pushing" (there is no good word for this) the direction of a dream in a particular way too hard will wake you up. It has to be done very gently.
* Abstract reasoning / logic is difficult in dreams, it causes you to concentrate too hard and then you wake up.
In general the limits are to do with the two dominant modes of thought: spontaneous unquestioning creation of ideas is very compatible with dreaming and you can float easily between multiple "perspectives" or versions without any problem. Analytical / questioning / critical thinking is incompatible with dreaming and causes you to wake up. Lucid dreaming is typically learning how to float in the sweet spot between the two modes.