Months ago I saw screenshots, allegedly for a WA prototype, that showed what could basically described as a plugin system for Whatsapp. So there wouldn’t be interoperability with the WA protocol but WA users could install plugins for other messengers like Signal, WeChat, Line, etc.

I don’t know what happened of that. When I saw the screenshots, I had a hard time believing that this would satisfy the EU because it was pretty much the wrong way around. Maybe they reversed course because someone from the EU told them “nice try but no”. Going purely from gut feeling, I’d say they could add RCS support, so that users without WA could be added to WA group chats as well as be contacted from WA, similar to for iMessage does it. Another path could be that they add interoperability with Signal because that is what WA is using for encryption anyway and other competitors are then directed to Signal implement their messaging service. Again: This is speculation on my part.