How would you do this?

[eluser]jnorris441[/eluser]
I am creating an online store system that will run many sites.

The approach I initially thought of was to keep the store core somewhere, and extend CI's Loader to use the core classes UNLESS there are classes specific to a certain site available.

Or maybe I could use routes to remap /category/view/1 to /mycoolstorecategory/view/1, so that I can have a mycoolstorecategory controller that extends the core category controller.

I'm already holding the views paths in an array that I can change per site, so that part is taken care of I guess.

How would you construct your app so that it can be easily upgraded but also customize the controllers and views per site?