The guys at Hidden Path are trying to use Kickstarter to fund the sequel to this great game. If you're interested, you can search "Defense Grid 2" on Kickstarter. They're close, but need about a push to reach the goal. If you log in to your DG1 game on Steam, it will offer to take you to the Kickstarter page.
They have also added weekly bonus challenges and such to help boost awareness. I don't wanna go posting a bunch of links and such because I'd rather not get this locked for advertising. I'm just a fan, not a dev or anything. But if you enjoyed DG1 and would like to see DG2 get made, consider checking it out and maybe kicking a few bucks their way. :)