The contents of this thread are sounding more and more similar to the Techy Thread over on the open forum every day!
It does bode well that in some cases, ECUs are "hackable" - not just for more power but I can see a future where some sensors will be NLA and the ECU can just be rewritten to use parts from other manufacturers.
A bit like the graphics card in your Asus PC failing, you just put a totally different one in and change the driver.
Losing the immobiliser is also good, no doubt there'll be insurance grumps if they knew but how many times do old cars get sent over the bridge for an immobiliser fault? Just pick and choose the features you want your car to have.
More likely to happen is that sensors will have adapter boards to convert the signal from a generic sensor to what the ECU expects. Already possible to DIY easy enough, just little need to.
Also likely possible to rewrite the firmware on ECUs. Especially not that hard, as one thing the diesel emissions scandal has highlighted is that Bosch ECUs are all the same firmware. There are constants and lookup tables that the manufacturers change to suit their needs but the basic firmware is the same. This is how remappers remove/map out the DPF and/or EGR valves.
Already with Saab ECUs the firmware has been disassembled, understood and altered.