One of the most obvious omissions from Dawn Of War were the Tyranids, bio-organic harvesters inspired by the Xenomorphs from the Alien movies who in turn served as the inspiration for StarCraft’s Zerg. New modes and forces are still being added and while the community isn’t quite big enough for you to find a multiplayer match without organizing it yourself, there are plenty of people playing through the campaigns and messing with the skirmishes.
Just have a look at its Mod DB page for proof. Even with its three expansions it never covered all the ground, which is part of why it’s still popular with modders today. It couldn’t represent the full ridiculous breadth of Warhammer 40,000, though. It pushed RTS into the territory of tabletop wargames, with a focus on throwing units into killzones, only with the computer rolling the fistfuls of dice. The real-time strategy game from 2004 minimized the genre’s emphasis on building a barracks that spat out one warrior at a time in favor of giving you whole squads to control, then replenishing them via teleporter so you didn’t have to jog reinforcements across the map to get back to the good bit, which was the killing. Dawn Of War wasn’t that impossible game, but it came close.
