I did this yesterday,
Sorry for the late reply.

Great idea qwasix

First results with adjustments and my interpretation at http://play.qwazix.com/grog/?p=204

You are welcome to comment.

First of all, thanks for the survey!

After reading the notes, especially about last part on storage and bluetooth I was thinking: what if control panel provides only basic support for storage and bluetooth, but the architecture of it is such that for each applet, before to run it it checks whether an app named <applet>_extended exists, and if so it launches that app instead of the default applet.
This way Nemo can be designed providing e.g. just a basic feature for storage applet, and leave developers to design something more complete.
Also, the concept could be extended to the point that, in cases like application management where only an external app will exist, control panel could even provide a 'stub' (via a configurable option "show all stubs" ?) to launch that app, for those who still prefers to reach it via control panel.

This is a nice idea but the first thing that comes to my mind is the possibility of multiple apps doing the same thing and the ugly "Which application to use?" modal of android. That is obviously bad design and must be avoided, not that any better way comes directly to my mind.

Why is it bad design? Because it's annoying, and if you check "Don't ask again" the way to revert is difficult to reach and completely beyond whatever a user could possibly imagine. (Instead of a default applications list or something, the user must go to app manager, find the default application for this action and click the super-ambiguous button labeled Clear Defaults)

Mutually exclusive dependencies is also bad design as the user may have use for both storage applications.

On the other hand, extendable control panel is cool, and we must give more thought on it. One possible solution is to keep the simple applet and list all the relative applications in it, the way Windows Phone list the applications that can edit a photo, after clicking edit in the gallery, but that adds a useless step.

Any ideas?

I think that if the alternative is to leave the user with just one 'basic' applet, to require that no more than one extension per applet is installed IMHO is not too bad, considering that it is not about extending a complex operation like photo editing, but it's just an applet.

Or, maybe it could be possible to keep more than one extension installed, but then there would be a 'applet extensions management' that works a bit like the 'Debian alternatives system' to define which of the mutual excluding extensions would be active at any given time.

I like the alternatives system in general, it doesn't give me headaches, it does it's job quite well, and it's pretty easy to configure. This is a general approach though, which should continue outside the control panel too. (Browsers, galleries, etc.) On the other hand I really hate when applications try to hijack basic functions, so this system must be securely opt-in, so no rude postinstalls can force the newly installed phone application to take over the default.

By the way, two new articles are over there @Grog.