@xor This reminds me of what #ipfs is supposed to do. An out-there idea might be a mastodon-like service that uses IPFS as the backend. In the eyes of IPFS, a timeline and even an instance -- a collection of timelines -- is just another immutable directed acyclical graph.
I know the IPFS folks want a *completely* decentralized microblog, but there is value in the trade-off towards federated instances, I think. (Namely, identity management and moderation/spam control.)