Port the Excel SDK to other platforms
The Excel XLL Software Development Kit describes an Excel-specific C API that is used to create user-defined functions and limited macros for Excel. The C API is widely used in financial and engineering application areas to create high-performance UDF add-ins for Excel. It is also widely used to provide back-end and cloud data access through in-sheet functions. The C API is further leveraged to provide an Excel extension model for other languages like Python (using PyXLL), C++ (using XLL+ or XLW) and C#, F# and VB.NET (using Excel-DNA, which I develop).
I suggest you port the C API (or perhaps a subset) to the other platforms where Excel is available. This would allow add-in framework developers like myself to bring managed add-ins or Python add-ins to all the platforms where Excel is used.
Porting the Excel C API would be much simpler than making a full COM object model and VBA runtime available on other platforms and other Office applications.
Making some the Excel XLL add-in model available will allow a new class of financial and other models to join Excel in running on all devices.
Office Extensibility Team