So first thing required is deciding what is needed and what is over the top. CEGUI is for in game GUIs, there is no need to try to replicate everything in Qt, Gtk and the like.
One neat feature would be the ability to resize LCs and their children would react accordingly. This will allow rapid layout development assuring that people don't have to hand arrange forms and related stuff. I still haven't decided if this should be the default behaviour or if I should make the current behaviour default. The current one is tailored more for procedural UI creation whilst the planned one is mainly for layout prototyping and editing.
The thing that I definitely want to redo is the Grid Layout Container (for more speed - this will allow MCL to be reimplemented with window based cells - and flexibility like row span and col span, rearranging columns and rows easily, etc... it will be a lot of work).
Any other ideas? If I break the API I want to break it just right
