Nick Bostrom has identified four broad possibilities for the long-term future of humanity (Bostrom 2009).
First, humans may go prematurely extinct. Since the universe will eventually become inhospitable, extinction is inevitable in the very long-run. However, it is also plausible that people will die out far before this deadline, from one of a number of existential risks.
Second, human civilization may plateau, reaching a level of technological advancement beyond which no further advancement is feasible.
Third, human civilization may experience recurrent collapse, undergoing repeated declines or catastrophes that prevent it from moving beyond a certain level of advancement.
Fourth, human civilization may advance so significantly as to become nearly unrecognizable. Bostrom conceptualizes this scenario as a “posthuman” era where people have developed significantly different cognitive abilities, population sizes, body types, sensory or emotional experiences, or life expectancies.
Bostrom, Nick. 2009. The future of humanity. Geopolitics, history, and international relations 1(2): 41-78.
An in-depth analysis of the four categories of possible futures.