Looks like the vast majority of plugin users only depends on core/pubhubsub.

Maybe we can set window.require that supports certain modules to keep backward compatibility. For example:

if (!window.require) {
  window.require = function(deps, callback) {
    const modules = deps.map(dep => {
      if (!(dep in window.require.modules)) {
        throw new Error("Invalid dependency name");
      }
      return window.require.modules[dep];
    });
    callback(modules);
  };
  window.require.modules = {};
}

// inside modules e.g. pubsubhub
if (window.require && window.require.modules) {
  window.require.modules["core/pubsubhub"] = { sub };
}

Will probably be longer than this, though... @sidvishnoi, does this idea look okay?

That's a really cool idea. We won't need to bundle requireJS this way.

window.require thing landed via #1981

I'm not sure we want to support "tapping" into any/all core plugin/hook. How about something like:

const myPlugin = {
  name: 'plugin-name',
  // some custom hook name instead of listening to pubsub events, or, "when": "after-markdown", "when": "before-markdown"
  after: 'markdown',
  // conf is respecConfig as usual.
  // utils is an object of methods as an abstraction over respec internals, 
  //  making us expose as little as required, reducing breaking changes in future.
  async run(conf, utils) {
    utils.showError(`${conf.unCool} option has invalid value.`)
  },
};

This follows pretty much same structure as present ReSpec plugins.

core/base-runner can do a pass where it collects the run order for all plugins in respecConfig.plugins[], and modifies the overall modules array. Using a plain object as plugin helps in keeping it simple/clean.
Internally, we might expose pubsub events to running plugins. Note that it's better to allow hooks only before/after plugin running, so they're not subject to changes in internals of core plugins and only depend on plugin order, which we limit by exposing a few hooks.

It looks promising! Probably needs some experiment by converting existing specs to make sure that we have all required features.