Okay so, I have been thinking the following. I have worked for this company that is located worldwide, but their main market is located in 3rd world countries. Some of the users, have low connection to the internet and not very fast phones, so we had to develop our PWAs using plain vanilla JS. I know you can make pwa with react, vue and Angular, but sometimes plain JS is better ;) specially for the scenarios I just mentioned. In some of the projects, we were using Lit Element too though.
I saw polymer has a starter kit for PWA. https://pwa-starter-kit.polymer-project.org/. Polymer is good, but ehh fuck it.
-A Design system using CSS/SASS (We are going to need designers)
-The basic structure of a PWA.
-Basic files(service worker, manifest, robots.txt, etc)
-Basic PWA operations like file caching, Push notification setup with Firebase, social sharing, offline capability, etc.
-Make it accessible on NPM
-Make it typescript friendly
-Maybe have the security with a tool already pre-setup
-Having the option to add workbox to your project
If you are interested on this, send me a dm on twitter @devpato or, just comment below.