Second wave is free to download today. It adds a bunch of extra modifiers to the game (e.g. random soldier stats, increasing costs of satellites) that can be set when you start like iron man mode is. You need to unlock them though, you get four to start with and unlock 4 more for each level of difficulty that you complete. Looks like there's some interesting stuff there, modders found it ages ago but not everything was implemented fully. Here's the pc gamer article.