You can look on how designer runs the jsonPath expressions to find what the tasks data inputs/outputs and process variables are and then use that info if the plugins you are using allow for auto-completions.
If they don't it might be a little harder to implement, but idk.
Are you willing to contribute your work on this? I think it would be a good addition, and then we could together take a look on how to solve the autocompelte problem.
HTH