We already have a standard IDE in Visual Studio. We also have a very innovative IDE in WebMatrix that can work with node.js and php in addition to ASP.NET. Why do we need another IDE? Well, let me first talk about the feasibility and then throw in some ideas on what we could do with it.
So what can we do with a cloud-based IDE? Firstly, like what Cloud9 demonstrated, the code lives on the cloud. Secondly, people can collaborate. Thirdly, I think it is possible to improve experience to what we have never seen before, so let me elaborate below.
In Visual Studio, when we want to see how the ASP.NET page looks like, we switch to the designer mode but that is not really close to what we see at runtime. In a cloud-based IDE, we can see the code and how it renders side-by-side, like what JsFiddle has demonstrated. ASP.NET does not execute the code on the website directly; it first compiles the code into the “Temporary ASP.NET Files” directory and executes the assemblies from there. That makes it feasible to edit and run the code at the same time.
Visual Studio has been heavy on ORM (i.e., entity framework) and lighter on code-generation. I have been favoring code-generation over ORM. This is because with ORM we are tweaking a black box to generate desired SQL. With code-generation, we can see exactly what we get and tweak the templates when necessary. Visual Studio has limited runtime information; it gets its information either from parsing the code, or from meta-data stored in XML files. With a cloud-based IDE that is working with running code, it is possible to get richer runtime information and generate code (or scaffold) under much wider scenarios. So how do we map the server-side code to html in the browser? That is where ideas like source map could help, and we have very good tools in querying DOM already.
So I believe all the technical pieces for a cloud-based ASP.NET IDE are available.