I wonder if a more explicit graphical flow-based programming system would be even easier than the implicit flow-based setup that's going on behind the scenes in Excel. For example, you have an object that represents an incoming stream of quotes. An output pin on that object connects to a "formula" object, which is wired to an output stream object. It's all based on ancient ideas (like Max/MSP, Pure Data, etc.), but maybe it could be useful.
Disclaimer: I created a flow-based system for home and building automation (no doubt one of many), so I tend to see uses for it everywhere ;).
Explicit graphical flow-based systems have their own issues. If the logic is simple, the systems are manageable. However, complex logic very quickly leads to a very complex graph with huge number of lines criss-crossing each other, until they make no sense.
Conal Elliot has done some interesting work in the area of "visual programming." The following is a good paper, but entirely academic: http://lambda-the-ultimate.org/node/2159.
Disclaimer: I created a flow-based system for home and building automation (no doubt one of many), so I tend to see uses for it everywhere ;).