If you like Warhammer 40,000, have I got the mod for you: Unification Mod for Dawn of War: Soulstorm is a sprawling remaster, bugfix, faction-adder, and game mode adder that works for both the Steam and (to my delight) Actual Physical DVD versions of Dawn of War: Soulstorm. The unification mod adds new art and units for the nine base game factions, as well as 17 new factions.
The big selling point of Unification is the new factions, but realistically you'll get a lot of mileage out of improved AI, as well as big updates in the campaign and new win conditions usable in its missions. There are also new Survival, Last Stand, and Kill-Team mission types. Survival and Last Stand are particularly appealing to those who prefer their RTS to be PvE and cooperative.
You might think, what the hell, 17 factions is... too many? To which I will say: There are more flavors of Space Marine than there are flavors of ice cream. Here are the new factions: Dark Angels, Witch Hunters, Adeptus Mechanics Explorators, Emperor's Children, Inquisition Daemonhunters, Imperial Fists, Salamanders, Steel Legion, Harlequins, Thousand Sons, Legion of the Damned, Thirteenth Company, Blood Angels, Chaos Daemons, World Eaters, Black Templars, and Tyranids. Of course, there are mods you can use to add even more factions, if you like.
Anywho, you can find Dawn of War: Soulstorm's Unification Mod on moddb.