What are the chances we could see MCU3 with all the connectors for both mcu1 and mcu2 retrofits also all the IC's to make it work on anything?
I think this would come closest to being feasible and pleasing everyone. A new a bunch of new cool features could be triggered by an “MCU3”, making even Model 3 owners want to update. Could be designed from the get-go with upgrade from MCU1 and MCU2 in mind. Tesla could first cut over all manufacturing lines for all models to the MCU3, then offer the upgrades once the dust settles. With lots more pull from lots of new features and current M3 owners needing it to get those features, market would be much larger for the upgrade, might be enough to make it worthwhile.
I can dream.