Low Level Abstraction Layer:

For the use of 2D GL Interface routines within the plugins and the gameengine. On this Level some Primtiive Drawing Functions are directly available, though plugins can have interactuive render previews, or specialized Views for the Application.

High Level Abstraction Layer

A workover of the current python gui interface, to make plugin creation easier.

GUI Editor

Themeable or not, a gui editor, that outputs some easy to reuse gui configs for usage with plugins and scriipts would be great.

Editable QUI's

To say it with Alan Cooper, "We don't want no Dancing Bear Ware". So a complete themeabiiity is not a way to gain usability.
I would recommend a very easy way to chage shortcuts as well as Menu Content.
Multiple Shortcuts to functions, and automatic decision through context.

Increase Context Sensitivity

The Menues should shorten according to what i currently do, not blending options, put putting me in the right context menue.

History and Recording of Commands and GUI Actions

As well a more userfriendly undo, or better a live scripting/recording option in two modes would be great. One to create Makros, and Sample them into a tools Library, and a Second in which i can playback and record actions for showcases, which would be a great feature to great Tutorials in Fly throughs of the application.

Intenral Versioning

To integrate the file editing histry into the .blend file, to make it possible even after saving to undo.

Extend Usability of Scene Graph

The Scene Graph should be extended with a lot more functionality, like direkt editing calls, object relinking, and the like (multiplikations, copy/paste, transforms).

General Curve Copy and Paste (with 2d-3d mapping)

There should be a general possibilty to copy paste curve between the modelviews and the animaiition views. This way also motion catpture via rotoscoping would be possible.
The user should be able to save and load every curve seperately.


Floating Window(s) for extra View

Like Message Boxes, Input Floater, or a Floating SceneGraph, or other View.
Should be usabel for plugins also.


Easy Layout Containers for plugins

There should be a set of Layout Container for easy Intaerface Creation for plugins.


Customizable Tabs as Tool and Library Container

The Tabs should also hold user costumizable Functio, Tool and Library Elements. Maybe in even more than one Custom Tab.


....


There is more i can think of ... hope its of use ... then i'll post more l8ers

About floating windows:

You can use Ctrl-LeftArrow or Ctrl-RightArrow to instantly switch to other screens plus you can add as many extra screens as you want.
You can also use + and - to zoom in and out from some things, like the button view. If there is any significant wasted space anywhere you can fill it up with something else.

Floating windows that are used temporarily - like some of the Display Buttons (e.g. Game framing settings) are often a good idea I think.