From a VERY general standpoint, the best solution with the highest adoption-rate would be one where we have a custom solution for every car in that car's original ECU case with the stock connector in the stock location. Most rusefi users will want to have an installation in their car as easy as possible. From personal experience I know that I'd rather be sitting at my workbench, routing wires on the jumper board in the rusefi, than trying to route new wires in my car. So, the more we can keep stock inside the car, the happier I would be.
How can we get close to that? Of course, if you have a '89-'00 Miata, you're already 99% there. I guess if you have one of the Toyotas (or Hondas?) that currently donate their ECU cases and pigtails to the rusefi cause, you're also golden. But anything else will require some more adaptation.
If we look at the MS-Pro concept, there is maybe something for us to learn here. It's a board with a full feature set, sufficient to operate 95% or so of cars that are looking to go aftermarket ECU. This module can either be had in a bespoke enclosure with high-quality, even water-proof, connectors for people who don't mind doing some re-wiring on their car. That's usually racers. Or it can be placed on a motherboard that can be custom designed for different cars/ vehicles as a direct-ish OEM replacement. With this design you have the best of all worlds.
I agree with the general idea of a more modular design. It doesn't have to be 100% modular- because, as @
has mentioned, this may create some trouble with cooling and large currents going through connectors. I would also vote for soldering the modules for production, instead of spring-contact connectors.
Maybe we can have a certain number of high-current low-side drivers on the main board for the true high current circuits and all the rest can comfortably fit on modules. All the inputs are low power, high-side drivers are usually low power, etc, do no worries there.
Your standard 0.025" square post header can take about 5A per pin, so we should easily be able to stand up our 5V power supply. Maybe use 2 contacts for each- 12V in, GND and 5V out.
For the I/O module connection to be universal you would need to supply to each module the following (I think):
- 3.3V (could be created on the module from 5V when needed, if we want to use less connections)
- Power GND
- Signal GND
- connection to brain
- connection to jumper rail / main connector
So the power supply module would have to be special, but most I/O module connections could be fairly generic, maybe just offering a different number of I/O. Have modules with 2,4 or 8 I/O. Also have some modules with more power GND connections for injectors / igniters or other high-power low-side shenanigans.