The question would be something that goes like this:
What would happen if a large percentage (greater than 20%) of all organisms on Earth die?
The intent is to get general feedback on the impact of the hypothetical event, in the case of the example, the impact of the mass population reduction.
A more focused restatement of the question may be:
How would a global reduction of all organisms' population affect the oxygen level of Earth?
What will happen if a mature ecosystem loses 20% of all its inhabiting species in a sudden event?
Would any of the above be adequate for Biology S.E.? What may prevent the questions from being acceptable?--the implausibility of the hypothetical events or the scope of the speculations elicited? Or maybe any questions that elicit speculative answers based on hypothetical events are categorically impermissible?