Support restartable sequences
Restartable sequences are available on Linux, and would allow better spinlock performance, and possibly make it easier to move parts of the futex API to userspace (because atomic hashmaps are hard without using spinlocks at least somewhere).
This would likely be achieved by userspace providing its TCB page to the kernel. Such a page may also store sigprocmask and possibly the pending mask/signal arguments, if most of signal handling is moved to userspace.