In futurists' parlance a 'possible' future scenario -- as opposed to a 'plausible' or 'probable' one -- is a scenario that is very unlikely but is not prohibited by currently existing knowledge of science.
This is the least likely 'level' of scenario and serves as sort of a minimum 'likelihood' test for a futurist's forecast. In the Futures Studies field, the concept of 'possible' draws the line between a futures scenario and science fiction.