Hard.

Very hard. Especially because you first have to pick something as well installed and supported on hosts as PHP to even get traction. Good luck there.

Then you need to be at least as good in most or all features on launch to get any mindshare. Hope you have a few years to work on this side project before you release...

There have been plenty of attempts and of them all Ghost is about the only one to be taken seriously it seems and it’s niche at best (and not yet at the same level of features or ease of use & configuration).

So yeah, the answer is hard. Harder than you think.