I would like to have a way to allow certain groups (e.g. admins, mods, maybe some non-mods) to add tags to other peoples' topics. I know that this is already possible for moderators by editing the post, but this is neither very convenient, nor does it work for non-mods.

Ideally there could even be a way to allow everyone to add tags, which then have to be approved by a moderator.

Are there any plans to include functionality like this and/or any hooks/apps that make this possible?

The Simple Tag Management hook does some of what you want, and is very simple to modify (it's a very small hook). It won't get you everything you want, but some of it anyway. We have it installed at eGullet and it works well.

That looks perfect, thanks!