NTP is a surprisingly large piece of software considering it's narrow task... and at this point rather crufty, ... e.g. in spite of having a ton of monitoring code that keeps turning up vulnerabilities, it's rather hard to monitor.
It has complex filtering, but still can perform rather poorly (esp with broken servers available). etc.
And accurate and (increasingly-) precise time keeping is mission (and, in some cases, security) critical to many applications.
Time really should be a solved problem but sadly it's not. Though fairly accurate temperature compensated oscillators are not terribly expensive, they don't find their way into typical server clocks. Many public ntp servers give bogus data. etc.
Meanwhile, kernel timekeeping has become more sophisticated. I suppose one of the principles in varnish was letting the kernel do its job, so there may be some opportunity to apply that principle here too.
So I think there is plenty that PHK could do interesting here while keeping the codebase quite focused and compact.
I do wonder how he would compare this effort to some of the other NTP alternatives (in particular chrony).