One use case for blockchains I haven't seen yet: block trades.

Imagine a buyer or seller wants to transact a large amount of crypto but doesn't want to do it on an exchange because of slippage or counterparty risk. What about a decentralized facility where (a) buyers/sellers submit a request for quote (b) liquidity providers respond and (c) buyers/sellers either accept liquidity provider quote or counteroffer. Smart contract guarantees the requisite amount of coin is held by buyer/seller and liquidity provider and processes transaction if/when agreed upon. A trustless, self custody solution to block trades applicable to any onchain asset.