What are your goals with this?

Speed up development in current project, share code between internal projects or share externally?

First goal would be to get a clear code structure, the second goal would be the possibility to turn on and of modules and to share modules between internal projects.

One idea could be to create then different nuget packages for each module that you can add specific to sites. Blog with example on how to create a nuget package: https://world.episerver.com/blogs/Alexander-Haneng/Dates/2011/8/How-to-create-an-EPiServer-NuGet-package/

Regarding views in general I would suggest looking into MVC Areas if you want for example create a folder for the module that contains models, controllers and views.

I would agree with Daniel this time - it's important to understand goals you wanna achieve. and then find tools to make that path ;)

Ok if it's only internal I would skip doing Addon at least. Episerver Addons are useful if you want to share with others but too much work otherwise.

For code structure I would start by using the SOLID principles as far as possible to make the code itself modular and extensible. Find a balance between generic modular solution and keep it simple. If you don't like the default setup for MVC I would recommend checking out feature folders. There are some blogs about that and Episerver.

Next and last step is actually sharing the code. Copy paste is usually what it boils down to unfortunately. I've done a few nuget packages too to use internally. Did it save me time? Nope :) Was it cool? Yes.