We’d like a lot of things, but may not get them (especially new Missions). But still, one can hope!
Challenges would be different from Achievements since the latter is built into the regular game. The former would be single player levels with specific maps and scenarios in mind. It’d be new material, but only having to design for a single player should simplify things.
You also can’t rely on specific missions, or your ally helping you out, which makes certain achievements much easier to do. Some of them could also double up as training
Stuff like:
Karax
–Beat the level without making a single combat unit
so it’d be all SoA and towers
–Beat the level with the given units
you’d need to keep them alive, properly micro them, and use SoA and towers for support, while pushing
–Take out cloaked and burrowed units with just SoA
Zagara
–Fight off as many air waves without using Scourge
Tychus
Survive as long as possible while defending multiple areas
etc.