what should be the structure for big web application in mvc asp.net

We are making a big web application and we will continue to add modules in future to the application after first release. Modules like inventory, Purchasing, accounting, payroll etc. Any addition of new module should not affect existing working. and tester are supposed to test only new addition and modifications. We have number of developers and they are supposed to work on their own module without effecting other developers work.

Once we think to create Area in main application for each module and then create separate dll's for models (business objects). so that controller names does not conflicts with other module. In this case all modules can share data between each other as all belong to single web application. But the problem will occur in future when we will add more and more modules in main application. In this case main application may become heavy in future and difficult to maintain.

Then we think to create separate web application for each module and create a main web application where links will be given for all web applications. But in this case we don't know how to pass data/objects to other modules as they are in different web application

Please give me suggestion what should we do. If you have any better suggestion or reference then please share