Yeah that's fine guys. I see exactly what you mean. On the other hand, let me answer to certain quotes.
If you really want, I'm sure we can dig up some code that would add the user as admin for all sites on your network. I don't know about YOU, but I don't want that.
This is exactly not what I want. I want ONE user to be admin on ONE subsite (child site). If I want to enable it on ALL site, I will change it's account for an admin one.
Also, remember that installing plugins requires knowing the FTP id/password to your site. You really wanna give that out? I don't.
Installing plugins don't mean that you really need an ftp account, and an ftp account can be configure to point to a specific folder for a specific user. The problem is that with the network configuration, each sub site is not another installation of the wordpress, so on the ftp, everything is in the same folder.
So no. You can't do this unless you trust someone enough to give them the keys to the car.
You can see my situation exactly like that. I trust the user enough to admin their "subsite" (the child site if you want), but the master site is not their responsibility. So, it's like having a child that has its driving licence and you trust him to properly drive your second car.
Because if your user installs a plugin from somewhere that injects malware or dodgy code, or something they wrote themselves, or inadvertently picked up elsewhere, it can bring down your whole site.
I perfectly agree. but like I said, I trust them. They are with our company for years now. They will install plugins from the wordpress plugins repository and that's it. They are not developers.
But that's ok guys, you explained it to me, I appreciate and I understand. Of course there is a good security risk there. That's fine, we will deal with it. I gave you some responses here, because I want you to see that when you deal with honest people, this kind of feature can make sense.
In the case of an admin, the UI of the admin interface can be confusing when you can switch from the main site to another one. At the end, if you switch regularly, it can happens that you post something on the wrong site, or install a theme on the main site instead of a child site, and same thing for a plugin. That was another reason why I was looking for this kind of feature: being admin on a child site, and another role on the main site.