Improved shader based motion blur technique that doesn't hammer performance like construct's current method.
Event search filtering - a search box that when typed into, hides all events in the event sheet editor except the ones pertaining to what was typed in, like the windows search box. So you could type something like "sprite rotate" and it would only show events that have actions/conditions/expressions containing all of the words.
Set text size, font and icon size in the event sheet editor.
Exporters to mac, linux, XNA, PSN, Wiiware, DSiware, iphone, android, web.
Like Quazi said, full 3d camera and sprite position/angle control - I don't want construct to try to become a full 3D game creator, but I think there's a lot of possibilities if we had full control of the angle and position of the camera and sprites. It's a lot easier to move the camera through a world with multiple 3D objects than it is to correctly position multiple 3D objects around a camera properly. Basically the camera as it is is pointed straight out towards the horizon, so even making something simple like a 45� looking at a 3D world is made far more difficult than simply setting the camera angle. If we had the ability to do that with the camera, we would need to be able to aim the sprites at the camera as well.
Behaviors able to work on per-layer basis - so they could be set to collide only with objects on their own layer.
Real time compiling - I'm not sure this is possible, but it seems like it might be. The graphics I don't expect to be able to be compiled in real time, but would it be possible for events to be? I'm not exactly sure what happens on export, but if upon editing the event sheet, it simultaneously edited the compiled event sheet, then no matter the number of events, they would have no affect on the amount of time to preview. That sounds cool to me!
Better vram control - I have a lot of animations in my game, but most of the time I'm not using a lot of them. Some of them only get used once in the entire game. I would love to be able to set a sprite to 'manual mode' where I could tell it specifically what animations to load. As it is to get around vram issues, I'm having to create new objects and swap them in, toggling the visibility. It's very clumsy.
Gamepad support
Instead of Quazi's idea about 'expression shorthand', what if there was intellisense for commands and object names, same way there is for variables? So you would press 's' then a menu with sprite highlighted would appear, then press enter, then press '.a' and a menu with angle highlighted would appear. I second that request as well.
Alspal: not sure if you know this, but you can hide the ribbon by right clicking on it.
Shinkan: you can use the 3D object to import a sphere.