Modify default widget
If you need to change the way the default widget works, you have several options to achieve your goals without modifying ImpressPages core files.
Additional widget skins
You can add your own custom skins to existing widgets by just adding your own HTML representation of widget's data.
Duplicate the widget
If adding additional skin is not enough for you, try to duplicate existing widget. Default widgets are located in Ip\Internal\Content\Widget directory. Choose the widget you want to modify and duplicate its directory into your own plugin with different folder name. E.g.Plugin/MyPlugin/Widget/NewName. Then very carefully change namespace and all other places where old name of the widget appear. Don't leave any traces of old name.