I would like to see an Atan2 function, under System > Math, as per http://en.wikipedia.org/wiki/Atan2
Admittedly, this isn't, and shouldn't, be a top priority, only something to be added eventually. It's uses are fairly rare, from what I've seen.
The Wiki article states, "A principal use is in computer graphics rotations, for converting rotation matrix representations into Euler angles", which happens to be exactly what I need it for.
EDIT: Maybe I should try to help further the 3D side of Construct by doing some actual coding myself. Stuff like this I could easily add by myself if I had a copy of Visual C++. Might try to get my hands on a copy and see if I can't help out at all.