Dilution is a potential attack i randomly came up with in a Twitter arguement and couldn't find any references to or convincing arguments of it being implausible.
Suppose a malicious actor were to acquire a majority of hash power, and proceed to use that hash power to produce valid, but empty blocks.
As far as i understand it, this would effectively reduce the block rate by half or more and since nodes can't differentiate block relay and block production there would be nothing they can do to adjust difficulty or black list the attacker.
At a rough estimate of $52 per TH equipment cost (Antminer pricing) and 12.5 BTC per 10 minutes power cost we are looking at an order of $2 billion of equipment and $0.4 billion a month of power costs (ignoring block reward) to maintain an attack - easily within means of even a minor government-scale actor.