I thought this already existed? Isn't it how some of the 3pp projects out there now use it? Doesn't Masterplan use it?
It's been a long while since I played 4e or used masterplan but I was sure it used something like this.
There used to be an API for one of the earlier incarnations of the DDI compendium that people could use. I do not know if MP uses the API in its current configuration, or if the current Compendium still has the same API tools available.
What I would like to see for the Next version of DDI is something a lot more detailed.
Compendium - API that allows you to call up information, run searches (perhaps slowed so that external users cannot overwhelm and unintentionally DDOS the server, or on a separate server), etc.
Monster Builder and Character Builder - On the DDI website, very basic, bare-bones builders. No fuss, no muss, but also no flashy bits or anything. However, it would also have a detailed API attached to it so that external websites could also access it and run their own character builder which pulls the information from the DDI builder.
Note, in both of the above cases, people would still require a DDI subscription to use the external site, and the external site would require their application to be designed in way such that it does not negatively affect the servers by accessing the APIs inappropriately, etc.
Basically, have WotC provide the data as well as very basic tools, but allow for other sites to provide a deeper experience.