I'm just finishing one Revit add-in project, and I am ready for your add-in/app.
Your project description is not detailed, so I am concerned about opening file from URL part.
I am not aware that Revit can do such a thing. File can be downloaded to disk, and downloaded file can be opened, but not original file. Also all changes made, will be saved in that local file. I hope that this is what you had in mind :) .