I've seen this idea proposed countless times, but they always seem to either fail to deliver in terms of "FOSS" or turn out to be a kickstarter phantom project that never ships.
There was a post today, by one of the KDE developers who has been involved with the postmarketOS crew in getting mainline kernel support, i.e 4.16 and not with whatever fork Android ships on, for an off-the-shelf device (Nexus 5)