I would keep related classes together and in logical namespaces, e.g.:

Piwik\Widget\List - former WidgetsList
Piwik\Widget\Factory - former WidgetFactory
Piwik\Widget\Provider - former Widgets

I think it would also make sense to distribute this by domain, as @dczajkacc wrote.

There's one thing I dislike with the Piwik\Plugin\ namespace is that it's very easy to confuse with Piwik\Plugins\.

I would be +1 to move related things together as suggested, e.g. Piwik\Widget\..., Piwik\Report\..., etc.

We currently also have Piwik\Db, Piwik\Url, Piwik\DataTable…

I honestly hate things in the root namespace ;) I would definitely be +1 to reorganize things in separate namespaces (e.g. Piwik\Database\…). Especially since we have the old ZF1/PEAR convention with Piwik\Db in the root namespace and the related classes in a subnamespace (Piwik\Db\AdapterInterface, …). (keeping related things together is better)

However there's one advantage to keep the current [not ideal] names: it allows us to refactor the component later and use the good name without breaking BC. For example with the scheduler the old class was Piwik\TaskScheduler (IIRC) and the new refactored class was Piwik\TaskScheduler\TaskScheduler. That allows to keep the old class with the old name as an adapter to the new class, which is very cool because then we can write the new class however we want.

With Piwik 3.0 we could break the API and maybe move those classes although it could be quite a bit of work, especially to also keep all of our plugins compatible.

We can also set up class aliases or write subclasses to keep BC.

I honestly hate things in the root namespace ;)

That's a strong opinion :) I don't mind them there and find it can be even nice sometimes. I'd rather find Piwik\Date\Date or so not as nice when a shorter version (Piwik\Date) does it as well. A problem that I see with those classes is that they sometimes contain too many things and too many different things. Also there are some classes that rather belong into a different namespace maybe such as Piwik\Cookie could be under something like Piwik\Http etc. Saying there are cases where it's nice to have something under the root namespace but it's also nice to have all in one namespace.

How would we rename eg Piwik\Plugin\Widget under the Widget namespace? One suggestion was Piwik\Widget\Provider or basically Piwik\Widget\WidgetProvider. Any other ideas?

Hate maybe was a strong word rather "dislike" ;)

WidgetProvider could do but are we talking about the classes that plugins extend to define widgets? If yes then either each implementation is a widget (in which case the natural name for the class would be Piwik\Widget\Widget I'd say), or either each implement provides widgets (plural) (in which case Piwik\Widget\WidgetProvider makes sense)?

For now I put everything under Piwik\Widget. Eg Piwik\Widget\Widget, Piwik\Widget\WidgetConfig etc

Feel free to comment etc if someone has new or more thoughts on this