on opencv’s github, in the wiki section, there are “OE” opencv evolution proposals. you might be able to mine areas of (planned) improvement from that.
in terms of execution speed, there are already plenty of eyes on that on all the popular parts (DNN).
personally, I think the drawing primitives, line() in particular, could use a complete overhaul. accuracy (thickness when using LINE_AA) is an issue. I don’t expect them to be fast because they run on the CPU. I am a little disappointed that there’s no code path using OpenGL. that could accelerate all kinds of stuff in OpenCV, starting with line() calls and ending in drawing of textured meshes (either perspective or orthogonal projection), which is roughly equivalent to warpPerspective().