It's quite admirable that you're brave enough to do this in Bash though I would recommend making it work in other scripting languages like Ruby instead.
Anyhow if you plan to organize your code into multiple source files then you might find this tool helpful:
Shell Script Loader. It would allow you to call your files elegantly like how it's done in most languages.
And you might try looking at other
projects as well for design concepts that you might find useful incorporating in your project.