Using a thin client I'd like to be able connect to a service where I can author geoprocessing tools using a modelbuilder-like GUI.
The datasets hosted on the server would be available for me to include as inputs. For example, a site like the National Map would let me write models that run against all the data on their site.
When I publish my model, I would set a price. When the user runs it they would be charged for data use, i/o, cpu - and for use of my model.