Formula Mashups
Idea is to provide a lightweight extension option to Excel  formula mashups. The short idea  allow users to create their own formulas for use by combining existing formulas. This allows users to have easier to use formulas without security problems code brings. The mashups can be saved in the spreadsheet directly (and expanded for backward compatibility) so no external dependency exists.
Suggestion for implementation: Have a mashup formula a (special) sheet  named ranges in the sheet are the parameters and Cell A1 is the result. It will allow users to put test data directly in the sheet to test their formula.. plus be an example. The sheet can contain data for use in the formula.
Advantages: Additional formulas can be made available without updating Excel. Users don't have to use VBA/Javascript or coding. Excel can be made easier for some users and usecases. Users can share formula mashups. Through evolution and natural selection the most useful and stable formulas can be incorporated in Excel.
Examples of type of forumulas:
1) Easier to use date manipulation functions (anything from data addition to more complex formulas like imagine mashups with holidays already added for a specific country  e.g. have NETWORKDAYS already reference a table with the right holidays )
2) Data driven formulas (e.g. have a formula that can give present value (PV) based on the monthly inflation rate of the US)
3) Probably most requests for additional functions can be implemented as a mashup of existing functions...
The user might also appreciate being able to see "recently seen formula mashups" in sheets he opened and pull the mashup into the current sheet. (This can be implemented transparently in the formula editor dialog box).
2 comments

Ewald commented
Yes, let's define macros. I think something like that could be possible to have UDFs that are made of other workbook functions only; they should run in .xlsx files without warning, not requiring user confirmation, not needing to save as .xlsm !

Kenneth Barber commented