JavaScript MVC - review and suggestions
Since the entire app will have multiple modules I would like to split
  the code into standalone modules that can be loaded as required (any
  suggestions how to do this?)


Use a dynamic module loader such as requireJS. You can use it to load modules dynamically using AJAX, and it features an optimization tool to combine your scripts into a single file to reduce the number of HTTP requests on production server.


  How can it be improved?


Stick to shared conventions for the naming of variables. Do not use all-uppercase for regular variables, only when you wish to indicate a constant value.

Use a name specific to your appliation/website, organization or company instead of GLOBAL for the global variable referencing the root of your library, if any: it is no longer needed using Asynchronous Module Definition with requireJS.


  Are there any mistakes/issues I should be aware of?


You should think about the way you wish to handle the communication between modules. The latest trend is to abstract and decouple the communication using publish/subscribe pattern.

You can find more details in The Scalable JavaScript Application Architecture presentation by Nicholas C. Zakas, and you may be interested in the open-source framework that I developed based on this approach, which I presented recently in Paris JavaScript user group.