I wrote a koa plugin (thanks for the Dev server API), that handles proxy-ing similar to webpack-dev-server.

Is koa here to stay or is it still experimental? I'm asking before I "advertise" the plugin.

Sub-question / suggestion: it might be nice if upcoming Config file would support specifying custom plugins. This way, less custom code will be needed.

Yeah, Koa is most likely here to stay.

Also yes for specifying plugins in config files. Maybe we can even auto load plugins if they follow a naming convention (like vite-plugin-xxx)