I think your proposal is very good. The lang is good as well. The alternative is not so good imo, because it would require you to remove a separate file from /lang if you remove a plugin, instead of just removing the entire plugin folder. Also it will make installing the plugin slightly easier, as you can just upload a single folder to the plugins/modules dir and you're good to go.
Personally though, I feel there should be a difference between modules and plugins. It will not only make the transition less complicated (because you can keep the current plugins dir and you only have to move /system/core to /modules), but you will also keep the modules dir from cluttering. I personally think the line between modules and plugins should be drawn where there is a need for standalone page. For example, forums is a module because it has its own page(s), while recentitems is a plugin because it is only displayed as part of a bigger page (usually header or index), and thus does not use a standalone page.
I suggest the following structure:
- html (is this used ?)
- files (replaces users)