iClone is close to the dream previz tool and is definitely number one currently on the market, so I will take my shot to summarize points where I would improve it if I could - to make it really the best previz tool ever:

1. GUI: the weakest point of iClone imho. Gui is clumsy and ancient, pretty unusable. So:
--let me detach windows (content manager, scene manager, modify panel, mini viewport) to effectively use two monitors
--let me create more than one (detachable/re-sizable) mini viewports, so I can see different cameras at the same time
--let me see camera switch in mini viewport
--let me assign my own keyboard shortcut to any gui command

2. TIMELINE: second weak point, far from perfection. Usable only for basic timeline operations. So:
--let me edit timeline curves like any normal 3d software
--let me select multiple frames / multiple objects on timeline and shift them together
--let me create dopesheet, let me use story tool, like MotionBuilder
--let me clean timeline from too many keyframes

3. SCRIPTING: good scripting tool would skyrocket iClone productivity. So:
--let me record any gui command (like max, maya, etc) and let me use this in my script (this would already release my work from million mouse clicks)
--let me write simple scripts, so I could use props, actors, etc. the way I want. Imagine: to establish or rearrange scene with many props, actors, effects, cameras, etc. would take one mouse click!
--please let me record repeatable tasks in scripts !

4. CONTENT MANAGEMENT: iClone is pretty good in this with drag'n'drop support and all, but I see room for improvements:
--let me create my own groups of props and other stuff
--let me save this groups (cameras, paths, props, etc) so I can merge them all into existing/another scene
--let me specify file paths - at least for custom folders (C:\Users\Public\Documents\Reallusion\Custom\...) so I can use some other drive for content and I can make project based content management

5. CONTROLLERS: this is my biggest extra wish !!!
--let me control the camera with game controller, joystick, 3d mouse,... Otherwise camera is too artificial, it is impossible to make good camera work without this !
--let me record this camera animation and let me edit timeline curves...
--Please take a look at "Craft Director Studio" Camera Tools for some inspiration.

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*****SOME OTHER IDEAS *****
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6. PREVIZ TOOLS: I want to use iClone for the serious previz job. So:
--let me create scene blueprint (orthographic view like a floor plan) with dimensions/measures
--let me print this blueprint with camera properties (angle, focal length, position) and any other scene information I want to be included
--let me add simple notes to blueprints, so director can write down some additional notes...