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Most importantly, I would document everything a developer would need to know in order to write a plugin for your script.
I would create an include file, which each plugin author would include in his plugin. This include file has functions/classes that talk to your script.
Make sure to document those functions/classes in your include file very well.
Document your database, so plugin authors know what each field and each table is used for, so that they can make use of them also.