If you are having lots of "programmable options" and say the ability to add and customise lots of inputs and outputs, then when you release the software package make it such that the end user doesnt need to learn C or visual basic to programme it. Ie simple interface for voltage or resistance based inputs or simple sensors. eg you could manually type in the graph data for say a resistance or voltage based sensor, label the parameters and then specify how it is used. Simple Switchable inputs could be easily controlled through the interface.

Might make quite a bit of work to have a easy to use interface but you can limit it to certain types of sensors that don't impact the map that severely.

Not just limited to engine controls either. Most OEM stuff is more of a "car control unit" rather than engine control unit.

eg fuel level input - for a guage.
gear temp/diff temp - linked to pump and or cooler fans - eg gearbox pump turns on at oil temp xx and fans kick in at yy and warning light illuminates at zz.

the usual water injection, anti lag, methanol control. boost control, launch control etc.

Maybe even a simple traction control based on ABS sensors or similar to detect wheel slip.

I think a really cool feature would if you have enough spare processing power and onboard memory would be a graphics driver that can be used to run a LCD colour dash unit.

Or perhaps develop an add on embedded microPC small "blackbox" that runs a cut down display version of the mapping software callibrated as a dash display. That way you can keep it seperate from the ECU but connected via the comms port. That way the ECU doesnt know if its mapping software or dash software (it doesnt need to know) as how the information the ECU is transmitting is used is a function of the software on the other side.

If you wanted to go really posh, the software package would have the dash display function as standard and then once loaded onto a micropc (like the ones you see in industry, marine, heavy machinery etc) then in theory a user could plug a small usb keyboard/mouse unit (plenty micro stuff out there for car PC's etc) into the car and use the dash display for mapping too.

ie if you buy the full ECU plus micropc dash display unit then no need for a seperate laptop for mapping.

Hell with modern operating systems you could even have a plug in second display option so the dash unit can display a dash while a passenger has a plug in display and keyboard to do live mapping.