A truly "next generation debugger" as replacement for gdb, lldb and the MSVC debugger which allows me to watch my program working in realtime (and pause/step/rewind at any time). Visualization should include things like a memory-read/write/execute heatmaps, a realtime flame-graph profiler, a data structure browser which "knows" the entire program state, and a much tighter integration of such a debugger into the regular edit-compile-test workflow.

As it is now, debugging is still stuck in the 20th century (I realize that the "realtime part" is hard to do, and may require to get the compiler and even hardware peeps on board too).