In no particular order :

* Some sort of menu replacement --

  Should be able to "tear off".  
  Items should be able to have a pixmap.
  Individual items can be "hidden".
  More robust.
  Better dynamic API than Fl_Menu.
  
* About window --
  
  FltkWindow :: Fl_About (title, version, copyright, authors, comments, logo);

* Fix docking stuff in Fltk 2.0 and add more "dock acceptors".

* Fl_Dockable_Group - needs a function to save and load the state of the 
  floating and docked items.

* Interprocess communication.

* 3D controllers (scale, rotate, translate)  Like Gleem.

* Themes and scheme support for Fltk 2.0.
