Frankly all I am playing right now is RA3. There is one thing in that game that got me addicted: Commander's Challenge. I really hope Blizzard can make something like this for SC2 in an expansion (although it won't be as good as this one in RA3, I am sure....)
The Commander's Challenge is a huge pool of seemingly disjoint missions. In most of the missions it seems to be similar to a normal skirmish with a computer opponent - but with some twists.
1) At the beginning, you can only build some of the most basic troops in each faction. Each time you win a mission, you "unlock" a new unit to build.
2) Each mission has a theme. Some of them are simple; for example during the mission which unlocks Attack Dogs, the opponent likes to build a lot of Attack Dogs. Some are hard; for example during the mission which unlocks Natasha, there are a lot of free vehicles scattered all over the map.
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This needs explanation. In RA3, Natasha is a 'hero' unit with a special ability that kills the operator of vehicles. After that is done, anyone can order any infantry units to board that vehicle, taking ownership of it. This makes Natasha a good counter against most high-tier, high-cost units. Basically, that mission has so much free stuffs I need to take that I had to maximize my APM for a very long period of time.... *crazy micro*
3) You can choose which race to use. Depending on how much you have unlocked and what the mission is about, this can be the key to victory; for example there is one mission which you are only allowed to build the basic tanks and nothing else; in this case Soviet is better because their basic tank is the most powerful, both statistics and ability wise.
4) At the beginning of each mission, the enemy's commander does some "trash-talking". After s/he is defeated, s/he makes some funny excuses.
Anyways, I am about 15-hours into it, but according to the game I am only about 20% complete. I am stuck on the Yuriko mission and the mission which I am given a fully-built base but have to crush three enemies who are allied against me.
I think I almost figured out how to beat the Yuriko mission. With Natasha I can kill 1 of the 3 opponents at the beginning, but I currently do not have Tesla Coils so after I kill one player, the other two will send some Yuriko(s) and roll me over.
Please I give me Tesla Coil soon...