This would a really difficult feature to implement I can imagine. Because of the "syncing hell" issues and possibility of data loss. This is also probably why Notion has never really tried to make it offline.
But I can see this being a huge competitive advantage against Notion if this succeeds. Out of all the syncing solutions I've seen, the one I love the most is actually Anki's because it gives the option to either keep the local copy or cloud copy. This shifts the burden of any data loss to the responsibility of the user. Also, having some kind of rollback like Dropbox does would be supremely useful.
I couldn't agree more with this. Talking about Anki, I have a lifetime account with Remnote which has an excellent offline mode while retaining many PKM features, so we know it's possible.