Work with the NSA, or an university or hire some contractor to audit important core pieces of Ubuntu such as the GTK+ and Qt libraries, X.org, and GNOME for security, bugs, crashes, etc. Memory leaks, buffer overflows, stack overflows, infinite loops, etc.
This will improve the security, stability and reliability of the whole operating system.
Improving the stability of system libraries (such as GTK+ and Qt) will improve the stability of all applications in the system.
And X.org needs to be more stable too, it is known to behave bad under heavy load, get stuck into infinite loops, have keys get stuck, etc.
