First of all, many thanks for being willing to contribute!
If you'd like to update an action or modify it slightly, you can just open an issue or submit a pull request.
There is only one thing I take as a "requirement", which is to make the change friendly to end-users where possible. If, for instance, you modify a Javascriptlet to use a new variable that the user has to set, please also add a "Set Variable" action to the task, so that it will be clear to the end-user that they can set it, without them having to look through the Javascriptlet's code. Other nice things to do are making sure that names are short (long names are a pain in Tasker on mobile), and that tasks mirror endpoints in the Toggl API as closely as possible, where appropriate.
Cheers!
- Christian