It's not hard to write your own Apostrophe modules. And it's not hard to let others subclass the modules you write.
But first, consider subclassing the snippets module. If what you want is a new data type with some fields and the ability to display them here and there around the site, that's probably all you need. Also consider extending the fancy pages module.
If you want to add a new widget, check out the RSS and Twitter modules as examples you can copy and learn from. Actually they are pretty good examples in general.
For the remainder of this article we'll assume you're doing something different!