Time is probably the limiting factor. Time is not a unit of measurement, it is a scope of perception. The human scope of time is basically nonexistent; humans are only able to perceive the present, so we use documentation to extend our comprehension of time. Furthermore Susan, the amount of time humans have documented is basically meaningless in the grand scale of the universe. Therefor, there is no reason that intelligent life doesn't already exist somewhere beyond humanity's limited scope of time. Other forms of intelligent life could even have a greater perception of time, being able to perceive the future or the past, in addition to the present. Maybe they're even able to perceive enough of our "human" future to see that intelligent life on earth won't exist long enough to reach where the others are located in time so they don't even contact us.
Functionality:
The main character/characters have a visible field of view. Inside the f.o.v., the game's time scale is constant. Outside of the f.o.v., the time scale is faster/slower. I guess I'm brainstorming lol, probably could've commented in the other thread. woops