The roadmap is an active area of research. The outline below is only intended to provide flavour, with more details in specifications (which have been released for phases 0 and 1 as of 14 Dec 2018, as linked to below). It is intended for this document to be maintained with ongoing R&D.
Sourced originally from the now retired spec for phase 1 sharding, with modifications.
- PoS beacon chain using Casper FFG for finality
- Validators create an RNG via RANDAO in block proposals
- Validators organize into proposers and attestation committees from the output of the RNG
- Validators create crosslinks for stubbed shards
- See the phase 0 spec FMI.
- Blobs (Binary Large Objects) are collated in shards without transactions (which require execution)
- Proposers submit blobs
- For more details, see the phase 1 spec and implementations.
- State-minimized clients. Stateless clients are not ideal as we don’t want to offload all storage into secondary markets, rather we can give people a choice to pay storage rent on the blockchain or pay for it in secondary markets.
¶ Phase 4: Cross-shard transactions: see here and more.
¶ Phase 5: Tight coupling with main chain security: here and more.
And a lot more: https://ethresear.ch/c/sharding.