Some of this may have been mentioned already.
-Improved stability. The program always feels a little fragile. The interface should be more responsive so it feels less so. An example is that the interface feels laggy when moving things around. Maybe it would help to use native Windows interface pieces? Another example: the event editor. Sometimes clicks don't register, or register as if you clicked somewhere else, and in general it just doesn't feel professional and almost makes you nervous that a wrong click is going to crash the program.
-Multiplatform support. Primarily I'd want support for Linux and Mac, but other platforms would be extremely nice (maybe the runtime should be easily portable so that more platforms could be developed?) I guess this would be mean the addition of other renderers like OpenGL.
-Maybe with the addition of other renderers as mentioned above C2 could do away with the need for that specific DirectX update? I find it really confuses casual users when they try to play things I make. (For instance, they insist that since they bought their computer after August 2008, it should already be updated)
-A better control system. Integrate the Xbox controller plugin to the main control system, add support for other joysticks and gamepads, and build in customization ala CustomControls.
-When a game displays fullscreen, there should be options to accommodate for widescreen monitors and the like. For instance, if a game is 4:3, the fullscreen resolution should be able to go to 16:9 so it doesn't appear stretched. Another example would be a game with a weird resolution-- if the monitor can't show that resolution, it should set it to one it can display and center the game in it. (For instance my game displays at 430x240, which is not a standard resolution-- I'd be fine if it could be centered in a 640x480 display or something) I know some people may say to do this with events, but there are many caveats involved in such a process that it's not worth it (for instance, it makes scrolling layers nearly impossible to do)
-The animation editor needs stabilized. It suffers from the same sort of issues as the main Construct UI regarding responsiveness. Also, there's the little filmstrip at the bottom, and you should be able to click back and forth through the frames using it instead of having to close out of the editor per frame. (I have a feeling it's supposed to do that right now, but it doesn't)
-Consolidation of all the audio options. You should be able to use whatever formats you want for music or audio. Additionally, the MOD plugin needs redone with a different, more accurate playback engine, which could then be added to the main audio plugin as well.
-Better support for application icons... I've noticed that you don't really get many options and sometimes it doesn't even save properly. Something more like MMF2 would be good, where you can make an icon for all the different sizes and color options.
-The debugger needs serious revamping. It is extremely unusable. At the very least, you should be able to sort the objects A-Z, because when you have a larger game with tons of objects it becomes a real pain in the ass to scroll up and down the list trying to find the object you want to debug.
Maybe more things if I think about them.