@cheong: Yeah, I misunderstood the issue here. I tend to glaze over when Death goes on a rant.
I don't know what's involved in Office dealing with macros, but yes, the document should be editable in as many cases as possible. When a macro is detected, you would need to firmly warn the user that any behavior that relies on macros will be broken.
There can be no VBA on the web or on mobile devices. It's too old and crusty, and the tools aren't any good. There would need to be some kind of superset to WinRT that Office documents could have access to, replacing VBA. The execution could be split into two parts. The "front end" could handle formulas and general editing. If you were running on a desktop, then the executable package could run locally, or could run on a server, much like Excel Services works today. On the web, this would continue to be the default execution method.
I'm not certain how something like this would work on a mobile device. Maybe it would be a balance of the two, depending on what would be running.