No offense, but it might be better to come up with your own ideas rather than stealing someone else's, and yes it is illegal since it's copyrighted (they normally just don't care enough to do anything about it).
I think coming up with something sci-fi that's
inspired by Star Wars, Star Trek, Stargate, Battlestar Galactica, and/or Dune could be pretty cool, but I'd avoid directly lifting anything from any of those franchises if I were you.