In short, the ability to write actions using shell script as well as todays node and docker.
Why? The containers that runs the actions has a lot installed that could very well be used in small shell scripted actions. That way the actions would be fast as they would need no download time.
Also I think this would be easier, or a good alternative, to work with than the node type. I find the commiting of the node_modules a bit cumbersome and makes the repos bloated.