What plugins would you want? My hope is to support most use cases for WP-plugins with built-in functionality.

That's actually a good idea. Last I checked, a simple out of the box WordPress install was not a good idea. I fully functioning wordpress site required you to at least install anti spam and security plugins if you wanted to use it in any serious capacity, along with a bunch of other stuff for basic functionalities now common to many websites.

I'm not sure, I write plugins when a customer needs it.

Stuff like custom forms, calculators, booking systems... For one customer I implemented a complete web hosting client control panel as a set of WP plugins.

Custom forms with support for triggering actions is definitely on the roadmap. I'm not sure where I stand on the possibility to add a fully fledged plugin system. I've been looking at the possible scripting environments that are easy to integrate/execute in Go, there's a few JS-interpreters for example. But it would be quite a task to make the UX good in the case of runtime errors etc.

It was a while since I had a look on how the plugin system works in Wordpress. I should read up on that. Thanks for the feedback!