One of the most viable proposed solutions to this existential threat is to establish a “leaderless” system in which the round is never started from the same validator or set of nodes:
Scalable and Probabilistic Leaderless BFT Consensus through Metastability - Oracles and Data - Smart Contract Research Forum
In effect, instead of “expanding the gap,” or “occluding the gap,” this method would effectively constantly “move the gap” so that an attacker’s target would never be the same validating block or set of nodes.