Blizzard are jazzing up co-op missions in StarCraft II [official site], adding optional 'mutators' to change missions in strange and difficult ways along with a rotating lineup of 'weekly mutations'. Each week, expect new preset challenges on a certain missions, such as the combination of infested hordes spawning in waves, infested monsters bursting out of corpses, and a deep dark fog of war for the Oblivion Express co-op mission. Blizzard are also boshing in a 'mastery' system for co-op, which will let max-level commanders earn more boosts to their commander's powers.
All this is coming in patch 3.3, as Blizzard detailed in a blog post yesterday. Yes, this will also all be available to players on StarCraft II's 'Starter Edition' demo, which includes co-op missions (as well as access to the custom game Arcade, remember).
Mutators will be optional, of course, for folks for fancy a challenge or simply to freshen up co-op. Available mutators will include enemies being fully cloaked, lava plumes erupting from the ground, enemies moving faster, and so on - small but difficult tweaks. The Weekly Mutations will combine several mutators in complimentary and challenging ways for specific missions.
For example, here are the mutations which will combine on the the point capture mission Lock & Load to become a git of a weekly mutation named Time Lock:
Speed Freaks – The enemy moves faster than any unit you have at your disposal, giving you little time to react to their tactics.
Time Warp – The Time Warp ability will be used against you, slowing the movement and attack speed of your units. These fields of time-manipulation must be avoided if you hope to succeed.
Mag-nificient – Mag mines will be scattered all over the map, forcing your army to move with extreme caution. If you trip their proximity sensors, they'll fly straight towards the unit that triggered them, causing terrible, terrible damage to you, or your ally.
So move quickly, but also slowly and carefully. Great. Thanks. Beating weekly mutations will net players bonus experience points too, which will be handy for the new 'mastery levels'.
Commander progression used to cap out at level 15, but mastery levels add a new way to boost your armies. Mastery points are a new progression track with options boosting power and abilities and whatnot. Players won't be able to earn enough to max out every mastery level option, though, so do think about skill builds before committing.
Here, look, Blizzard whipped up a video to explain it all and show a bit off: