For anyone asking about the Halo Wars DLC, there are two packs that come in it, one that has new game modes, and another that has new maps. I liked the map pack, since there was only 3 3v3 maps that game in the game and now there are 4. I haven't tried the other maps as I only really play skirmish on 3v3 maps. The map is basically two sides with one bottleneck opening that connects the two, so the middle becomes very action oriented.
For the game mode pack, there are three different new game modes. One is a Capture the flag esque game where you fight over taking control of a Forerunner Sentinel and then the unit that grabs it has to hold it (not die) for 2 minutes and then you get a point. The Sentinel then respawns and the round starts over again. This one is pretty challenging, as you both have to capture the flag and keep it from dying, while also fighting wherever else the enemy meets you, which can spread your forces thin very fast. The next game mode is a sort of a Tug Of War type game, where who ever has the most units tips the scale, and you have to try to take up the entire "rope" in order to win. This one was not as fun, as the game was practically over by the time I built up my forces. And the final mode is one where the computer gives you a certain amount and class of units, and you have to wait for a certain amount of time before you get more units. So you have to both survive with what you have AND go on the offensive to take out other with what you have.
All skirmish modes can be played with a co op partner if they want.