A very big one - and properly one that needs to be outsourced;
Rewrite the editor using standard elements from a stable crossplatform ui toolkit (GTK,Qt or wxWidgets ), this will get rid of many bugs and fustrations ( oh yeah that particular scroll pane cannot be mouse scrolled, scene tab will sometime stop drawing and just display the last tab, etc, etc, etc )
Many of those toolkits comes with a visual designer - GTK for instance has glade/gtkbuilder which abstracts the design down to an xml file, this file can then be edited independently of the functionality which in my book is pure win.
With the rewrite there really needs to be support for custom plugins and widgets (w. the glade approach you could even make custom panels ), as well as support for scripting editor actions in lua.
Oh and do consider using scintilla for the editor, no point in reinventing the wheel when a perfectly round one already exists and is ready to use.