Please make it something that helps me spot errors, and enforce better structure on the huge 2-d field of rectangles that has been in place since the 80s.
Not meaning to hijack the post, but I am currently working on a solution for error spotting in Excel- we really feel that this is one of the other huge problems with spreadsheets. Collaboration intrinsically leads to "lost in translation" errors (see JP Morgan's recent blunder) and these are difficult to identify, especially when you are not the creator of the spreadsheet. We are at http://www.slateforexcel.com
Honestly, I wish we'd see more spreadsheet-like apps move away from the "massive unstructured ocean of rectangles" and give us a little structure - something like Lotus Improv: