-A better lighting plugin system
-An advanced Sprite plugin that has slots for diffuse and normal maps and can take into account the lighting system mentioned above
-A path-finding behavior that work with the platform behavior (mainly for enemy AI)
-A timeline editor (I am way too deep in the magic forest from now on, I know)! Animate each object with keyframes and trigger those transitions with events!
-A true 2.5 camera system that offsets the layers in to z space, like the camera in After Effects, or Hitfilm, or Toon Boom products.
-A better audio system that doesn't loose sync, enables you to make buses and multiple sends, has fade in/out actions per individual sound or per bus.
-A rope behavior(!)
And some of the things that have already mentioned:
-Path creation system with bezier handles, easing in/out for each point, and access to all this with events!
-Better particles that support initial angle offset/rotation speed and per-particle collisions.
-Menu and Inventory plugins
-Text object that allows for multiple fonts/styles/sizes/colors per object
-A dialog plugin that advances the text based on user actions. Ideally it should support some basic "navigation" elements, like forward/back arrows, scroll bars, type writer option etc.
If not for C2 then for C3 Quazi...!
PS: I would very much like to see a well documented manual, with example (video) tutorials for Q3D first however