For Blender 2.8, we have made some changes in the UI which affect Addons. Here's an explanation of how we expect Addons to integrate with Blender moving forward. Depending on the type of addon, they can be integrated in different ways:

1: Active Tool Addons

In Blender 2.8, we have added the concept of active tools to all modes. In the past, this was only available in Sculpt and Paint modes. Addons can define one or more Active Tools now.

For many Object and Edit Mode addons, adopting the active tool system will be the appropriate solution. When the addon registers an active tool, it gets added to the toolbar, and a custom icon can be defined. The corresponding settings will be placed in the Tool Properties.

2: Panels Addons

Some types of addons can better be implemented as distinct panels of related functionality. These can be added anywhere in the Blender UI. For 3D View-related panels, then can register themselves in the 3D View Sidebar, like so:

3: Editor Addons

In 2.8 we have a new way of integrating addons, by letting them define one or more custom Editors. For certain types of addons, this will be the appropriate solution.

In Blender's addon system, each addon can use a combination of the above solutions, if desired.