How to add my Custom Content to Navigation Bar

I don't know if we have any good examples of implementing the plugins required, but here they are:

•  IGroupCustomNavigationPlugin (v2) Plugin Type  for custom group navigation items
•  IGroupDefaultCustomNavigationPlugin (v2) Plugin Type  to have a group navigation item be displayed by default
•  ISiteCustomNavigationPlugin (v2) Plugin Type for site level custom navigation items

  thank you, that was helpful!

I found, that 

ICustomNavigationItem

interface has property

ICustomNavigationItem[] Children

Is there any way to use that property? As I understand it may be something like dropdown links or I don't know //

I tried to add children links but don't see any UI changes.

public interface ICustomNavigationItem
  {
    ICustomNavigationPlugin Plugin { get; }

    string Label { get; }

    string Url { get; }

    Guid UniqueID { get; }

    ICustomNavigationItem[] Children { get; set; }

    bool IsVisible(int userID);

    bool IsSelected(string currentFullUrl);

    string CssClass { get; }

    ICustomNavigationItemConfiguration Configuration { get; }
  }

This is a limitation imposed by the Social theme - the maximum link depth supported by the Group - Banner widget is 1. Increasing it would be a customization (you would need to increase the maximumDepth parameter of the navigation configuration item and then update the link rendering in the widget to handle the .Children parameter accordingly).