We deliberately don't support this feature. The idea is that the build file should be self-contained, so have all the information to run in it, no external arguments should be required. It should be much easier for the final user to run a build. The tool however has limited parameterization capabilities via ./makesure -D VAR=value.
Includes
This is a considerable complication to the tool. Also, it makes the build file not self-contained.
Shells other than bash/sh
Less portable build.
If you need to use, say, python for a goal body, it's unclear why you even need makesure at all. Besides, you always can just use python -c "script".
Custom own programming language, like make has
We think that this would be unjustified complexity.
We believe that the power of shell is enough.
Parallel execution
makesure is a task runner, not a full-fledged build tool, like make, ninja or bazel. So if you need one, just use a proper build tool of your choice.