Thanks for kicking this off @jonringer 
My goal for 20.03 is a refactor of the httpd module, which I have started on. Currently I’m working on a PR to modify services.httpd.virtualHosts from being a listOf submodule (...) to an attrsOf submodule (...), which will allow for fun options that mimic nginx, like enableACME.
https://github.com/NixOS/nixpkgs/pull/73113
Currently the NixOS manual uses services.httpd for several examples. This module isn’t a very good example for several reasons, but specifically the Abstractions section gives examples which no longer are true with my change. I was hoping someone with better documentation skills than my own might consider rewriting the services.httpd examples in that section to use another module as an example. HELP WANTED 
Also, much like I was hoping for 19.09 I am still hoping the security.acme.certs module will be expanded/rewritten to support dns challenges, which @petabyteboy is already working on 
https://github.com/NixOS/nixpkgs/pull/63613
Aside from that… NixOS works really well for me these days, so nothing else is really jumping out at me yet. If anything catches my attention before the next release I’ll be sure to revise my post.