I have a project that attempts to improve the default AI by better emulating the current human metagame:
This project seeks to create an AI that provides a unique experience in each game while closely mirroring builds and unit compositions used by humans today.
This provides a much stronger learning experience for new players compared to the Blizzard AI. Players are encouraged to scout, learn what the AI is doing, and adapt instead of turtling up. The AI tends to avoid all-in strategies in favor of economic builds, but it also includes a number of rushes to make scouting and early defense important.
My goals are not to make the AI harder, but rather to make it more interesting to play against. I believe that this AI ends up being harder than the default in most match ups, but it could be more difficult if my goal was just to make it hard.