I really liked the idea of Helldivers and I'm glad to see there's still some activity around the title in the form of new content being released from time-to-time. Today, they've released some new free content titled "Democracy Strikes Back," which includes a new mission type and increases the max rank from 20 to 50, as well as some paid content in the form of new classes.
The new mission is called "Retaliatory Strike" and it will send you and your comrades into an enemy base to fight off waves of foes and liberate it. The only difference I can see here between this and the other missions is that it sounds like you'll be fighting enemies more directly in a kind of survival/horde-based fashion, as opposed to achieving set objectives around a map. If you own the game, or purchase it at any point in the future, you'll get this free of charge.
For just $2.99 each, you can purchase the new Terrain Specialist and Precision Expert classes. The Terrain Specialist allows you to move across difficult terrain (like swamps and snow) without suffering movement penalties and also comes equipped with a new strategem: Missile Barrage. The Precision Expert reduces the amount of time it takes for stratagems to be called, which is perfect for his new stratagem, the Precision Artillery Strike.