The two main types of add-ons that you might be interested in making are custom controls and external commands and functions (generally referred to as externals).
Custom controls are typically made of a group that holds many standard controls, and with a group script that manages the appearance and interactions of those controls. There is a tutorial on making such a custom control at this page:
http://www.runrev.com/developers/lessons-and-tutorials/tutorials/advanced-custom-controls/
If you intend to go on to make a nice custom palette to make it easy to drag-and-drop your custom controls onto the card window, consider making it DropTools-compatible. You would save yourself a significant amount of time solving the issue that DropTools already solves, and your custom controls would fit in with the other DropTools controls, making the screen less crowded with custom palettes! There is a lot of information on developing DropTools compatible controls at this page:
Down the right side of the previously-mentioned "lessons-and-tutorials" page is a long list of tutorials, two of which relate to making externals. Those particular tutorials are intended for developing externals for desktop applications. For developing mobile externals, you should look at this page:
http://www.runrev.com/developers/documentation/externals-sdk/
At the time of writing, this page only covers iOS externals, but there is reason to hope that by the time you are reading this, there will be information on creating Android externals too. The page is quite a long one, and even includes a 17-minute video! The video can be viewed on YouTube too, at this address:
http://www.youtube.com/watch?feature=player_embedded&v=lqduyQkhigg
Creating externals is not trivial. If you have an idea for a useful external, but feel it's beyond your abilities or interest to create yourself, take a look at the mergExt site:
Here you can make suggestions or vote for externals that you would like to see created.
If you do feel you can make externals, why not make them available as products? The going rate for these add-ons is in the $30-$100 range - any sales you make will offset your development costs.