Let's start a short project.
As engineer, I tend naturally to scientific calculators, but in the daily life we need to get into business more often than not.
You need to discuss the profitability of investments, their net present value, and several value of time issues. That is, IRR, NPV, TVM. While it would be good to have flows as long as you need, in my company all projects are calculated over 10 years - except acquisitions, and these are not discussed over a calculator.
You need to plan projects, and need to calculate how many days are left from now to then - that is, DATE+ and DDATE calculations.
Regarding %'s, the HP15c comes with % and increment%, but it misses the %T. It will be good to have a weighted average as well.
Depreciation is always linear - I do not need to make a program to calculate that. Bonds I will not use.
Black and Scholes would be nice to have - to show off. We do not use it in our industry. Instead it would be good to have optimal stock ordering.
I have not used it yet, but I see some linear programming coming - so I would rather keep at least room to solve a 3 equation linear system: 9 + 3 = 12 registers free for matrixes. As I said, let's use 1 register for initial investment, 10 registers for flows and 1 more for management (in addition of I as counter). This adds up to 24 registers, leaving 65-24 = 41 registers for programming. This equals to 287 steps. Let's see what we can do.
This is a project that will take some spare time. It is far from the complexity of the HP30b or HP12c+ repurposing projects being carried by some people. Nevertheless, it will be important if (when?) an HP15c+ hits the market.