This is something I believe phpBB should aim for in it's development. Currently namespaces are used with much of the code being divided up into small packages. I believe this process should be completed to include all phpBB code.

I understand this would be a major breaking revision and perhaps it is best for a major version release and could possibly already be planned for 4.0.0?

Currently many functions of the core phpBB system exist in the includes folder and live in the global scope of the program. Similarly several core variables live in the global scope ($user, $auth, $config.. etc). This makes it difficult to integrate phpBB into more complex systems where those programs also exist without expansive namespaces. Yes this is unrelated to the functioning of phpBB, but for example I am trying to integrate with a different software through an extension and $user, $auth, $config and $template are all also used by the other software. 

If phpBB could go completely object oriented with namespaces that would be ideal. It has been most of the way there since version 3.1 when all the Symfony libraries got added. It would also help phpBB push forward to be more compliant with the PHP Standards Recommendations (PSR).