Ethereum (ETH) co-founder Vitalik Buterin published an extended post outlining the Ethereum block building roadmap beyond the upcoming Glamsterdam upgrade, addressing centralization in block construction, frontrunning attacks, and transaction privacy at the network layer.
The post comes days after Ethereum developers officially scheduled FOCIL - the Fork-Choice Enforced Inclusion Lists mechanism - as the consensus-layer headline for the Hegota upgrade, targeting the second half of 2026.
Glamsterdam, expected in the first half of 2026, will introduce enshrined Proposer-Builder Separation (ePBS) directly into Ethereum's consensus layer.
That separates the roles of block proposers and block builders - but Buterin argued it doesn't resolve the risk that a small number of builders could still dominate and censor transactions.
FOCIL and "Big FOCIL"
The post describes FOCIL as the first step toward in-protocol, multi-participant block building.
Under the design, 16 randomly selected attesters each nominate transactions that must be included in the next block; any block that ignores them gets rejected by the network.
Buterin also outlined a more speculative extension he calls "Big FOCIL," where the inclusion lists grow large enough to cover all transactions, potentially reducing a builder's role to ordering only MEV-relevant transactions like DEX arbitrage.
FOCIL is controversial. Critics have raised concerns that forcing validators to include any valid mempool transaction - regardless of OFAC sanctions status - could create legal exposure for node operators and increase protocol complexity.
Encrypted Mempools and the Network Layer
Buterin also addressed so-called "toxic MEV" - sandwiching and frontrunning attacks on users - as a distinct problem requiring its own solutions. He highlighted encrypted mempool designs as one approach, where transactions remain hidden until included in a block, removing the window for hostile wrapping.
He separately flagged the transaction ingress layer - the path a transaction takes from a user's wallet to inclusion in a block - as an underexamined attack surface. A hostile RPC or analytics node can see and exploit unencrypted transactions in flight.
Buterin pointed to active research including Tor-based routing, custom Ethereum mixnets, and low-latency designs like Flashnet as candidate approaches.
Both areas remain open research problems with no finalized implementations on the roadmap.
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