The new Microsoft Excel API has its own built-in chart resource, allowing you to drive visualizations from the spreadsheet API. I'm sure the suite of business focused visuals they provide by default will meet a few of the common needs of the average business user. However knowing the appetite of the average excel users for charts, graphs, and other visual eye candy, I predict someone is going to do well if they get to work providing a robust, plug and play, API-driven visualization solution on top of the spreadsheet API.
The leading analytics players like Tableau will be quick to serve the space, but I think that a more open approach using D3.js would do well. Don't go thinking you have the latest startup idea please, but I bet if you start crafting a set of D3.js visualization solutions that worked easily with the Excel API, I'm guessing you will do well--especially if you start looking at delivering some niche solutions, beyond basic charts, which D3.js is wells suited for.
I recommend taking a look at OAuth.io for the authentication portion of it and deploy each solution as an individual Github repository. OAuth.io lets you handle the OAuth dance 100% client side, the Excel API will provide the data, and D3.js embedded on HTML pages will provide the eye candy. One of the most frustrating portions of reverse engineering D3.js visualizations is the data connector layer, and if you swap out with a seamless Excel API browser--game over! #winning This is your revenue generator.
To help make the Excel API, D3.js visualization be more embeddable and shareable, I recommend providing a caching option that would take a JSON snapshot of the spreadsheet data and allow it to be shared via link, embedded with a copy / paste, email, or any other common channel for collaboration. If done right, this embed and social sharing strategy would act as the marketing, and word of mouth for the visualization solution.
I am just sharing some of my thoughts, as I'm playing with the Excel API some more. I am not in the business of acting on most of these opportunities, and would rather articulate them, and put them out there for others to step up and provide a solution.