Can a hypothetical question be chosen as a research question? Suppose, can I choose "what would happen if the USA doesn't exist" as a research question? Would this be still acceptable by a wider researcher community?
At least in the hard sciences your question would not constitute a good research question. A scientific theory is considered a 'good' theory, if it makes falsifiable predictions (i.e. predictions which can be compared to observations of the real world) and has not been falsified despite comparing it to lots of observations. 

Suppose in researching your question you come up with the theory "Hotdogs would never have been invented if the USA did not exist". This is not a good scientific theory, since it can not be falsified by comparing it to observations.

Things would be different, if it would currently not be known whether the USA exists or not. If in this case you can logically argue that "Hotdogs would never have been invented if the USA did not exist", this would constitute a valuable scientific contribution, since it would provide a link between the observation of Hotdogs and the existence of the USA.