My first thoughts were along the lines rederikus mentioned. Since there is a web interface, seems the user could access a list of plugins, click on one and have it installed (and also uninstalled). Having some programming experience I believe this would take a fair amount of work to implement, but I think this would be a good goal to shoot for.
In the short-term I like makeself. While it takes some manual steps I doubt many users will be going nuts installing many plugins so the occasional manual process isn’t a huge barrier – folks had to do a bunch of steps to get the RasPi setup and interval running.