While I agree with you, the first step for user centric Android flavors regarding security is to support relocking the bootloader, with a custom (preferably the user's own) digital signature. As long as we dont have that, an attacker could flash or just boot a custom bootloader through fastboot that does its own thing.

However that doesn't really depend on Android system developers, I think, as the problem arises from the inferiority of almost every phone's bootloader (chain) (because most phones does not support setting up a custom signature for bootloader verification), and probably that can only be reasonably solved by device manufacturers, because as I understand, bootloaders do a lot of heavily device specific things, so there cant really be a common (primary) bootloader, and making one for each phone is a lot of work that also involves lots of reverse engineering, and maybe the early bootloaders cant even be overwritten on some phones..