[abendstolz]

Sounds interesting but at the same time a bit complex.

I assume you wouldn't ship the whole plugin runtime for each plugin that wants to host another plugin?!

[zamalek]

It's not that complex. You basically want to have all plugins support describing themselves (as a list of plugins, which is nearly always 1 item), and "activating" themselves according to one of their descriptors. Bonus points for deactivation for updates (this is often extremely hard to pull off because of how instrusive plugins can be). You could then have a utility header file or whatnot that does the [very minor] plumbing to make the plugin API behave like a single plugin.

Shipping the runtime is another good option, because it means you don't have to worry about runtime versioning.