Google's Project Treble was created to help fight Android's dirty f-word (fragmentation), by making the update process easier and faster for OEMs. Separating vendor-specific code like SoC drivers out from Android itself was meant to help when it came to OS updates and the work required to push them out. Now Google is working on increasing just how modular Android can be with something called APEX.
Details for APEX were published by well-known Rootless (Pixel) Launcher developer AmirZ on Reddit after he compiled together a bit of research on the subject. APEX was first spotted last year by Lawnchair developer Till as its own repository at AOSP (the Android Open Source Project).
At a technical level, APEX has been compared to Magisk, which works by mounting folders into the system partition at boot, rather than modifying the system partition directly (which is detectable). APEX appears to extend that same functionality over into core Android packages, separating out things like the Android Runtime into their own packages, separate from the system partition. That means they can be individually and separately updated from the system image.
APEX could take this a step further, breaking the OS framework into more parts.
It's possible that modularized OEM modifications could then be distributed on top of a Google-maintained system image — basically meaning the version of Android itself on a given phone could potentially be updated by Google, but the bits responsible for an OEM skin could be present, updated, and maintained as separate components. That's not to mention how it could ease ROM development, as Treble has.
APEX hasn't actually made its way into Android yet — I'd bet Google changes the project's name before it ends up in any official announcements or press releases — but its presence at AOSP and Google's Git indicate the feature is under active development and could land in a future version of Android.