Welcome to the Forum ...
I've seen a few devices out there and had the same thought myself, and not one to deter exploration of ideas, as I'd be keen to hear how you progress and the outcomes you achieve, I found that there are limited things you can use to "substitute" and fool the ECU ...
- temperature (a simple variable resistor - so no need to buy a special tool - search ebay - £5-£40 depending)
- Pressure - can be amp based (4-20ma) or voltage, voltage is simple - bench power supply or simple DC-DC buck converter (about £5 on ebay)
- PWM signal - again plenty boards on Ebay, (see a post by STC recently) from £5 without display, or about £12 with display of %PWM
However things like crank and cam, can't really be simulated, as you could not tie them to the phases externally, so you're limited to what can be simulated for use to test the car ...
I regularly build custom looms to drive components, for instance roof motors while utilising the inbuilt micro switches, with overrides, so we can cycle a roof either in the car or on the bench, or for the 575 Super America a loom included a external piggy control for chromatic darkness and OBD connector to allow the diagnostic to be plugged in to monitor or active/override the inputs of the car or my switches ... but each one is so unique to test/cycle something that the manufacturer failed to provide, and apart from the 575 which there are only a hundred or so cars built (but a roof that could cost £20k+ if mis-diagnosed, or worse blown while trying to diagnose), is for models we see regularly ... something most general garages don't have the benefit of!
So yes IMHO I think there is a need, but when you crunch the numbers, there are few general/generic applications, and based on costs, for us at least we tend to make as many dedicated "tools" as generic ones, and the generic ones can be made for far less than anything i've seen commercially ... plus you need to understand how to use them ... if you do, then you probably could make it yourself ... and you really should know what you are doing with substitute resistors or voltage generators as it's easy to blow ECU's
I'll say one thing, if you don't understand Ohm's law, STOP
but, as I started, don't be put off, and I'm more than happy to share my experiences and learn new tricks, that's the whole point of a discussion forum ... constructive and nurturing solid debate, helping people see the risks before they make a mistake, or find a useful new technique or trick to help them ... bring it on
Were there any specific things you had in mind? Sensors etc?
Commercial products you'd seen?