What changes should be to made to Linux if we ignore the "don't break userspace" rule?

Sometimes, breaking changes are good. However, the Linux kernel has a hard rule of not allowing patches that would break userspace.

Personally, I think that processes starting with 'X' should not be treated differently, because it just seems silly to me. Reference: https://www.phoronix.com/news/Linux-DRM-Process-Start-With-X

Which changes do you think should be made to the Linux kernel (that would break userspace)?