Mechanicus was one of the better Warhammer 40,000 games to come out in recent years. Squad level turn-based tactics with a layer of exploration on top, it pitted cyborg cultists of the Adeptus Mechanicus against the skeletal, robotic Necrons.
Patches improved its difficulty, but one thing still holding it back is the tight theming—one faction against another can get tiring after the first 15 hours of a 30-hour campaign. Developers Bulwark have a solution in the Heretek expansion, which will add renegade tech-priests as new enemies.
Heretek will also add a new tech-priest class called the Xenarite, who use forbidden alien technology, two new troop types called Infiltrators and Ruststalkers, more weapons, missions, and combat that takes place on board your spaceship, the Ark Mechanicus.
Altogether it sounds like something on the scale of the Enemy Within add-on for XCOM, and fingers crossed it can reinvigorate the base game as much as that did.