Well if this is for a game, you should probably think about what is going to appear and be important in the game. If it's a FPS, you might want to flesh out the various weapon manufacturers and different kinds of weapons in your world. If it's a city builder then the different cultures and architectural styles would be useful.
For alien life forms, depending on how in-depth you want to go, Biblaridion has some neat videos showcasing their process for creating alien life. Unless want to start a whole-ass spec-evo project you probably shouldn't go into this much detail, but the videos should still be useful to learn what to consider.