A not unthinkable scenario is that the Bitcoin price decreases to, say, 10-20% of today's value. This would make mining less profitable, and the difficulty should adapt accordingly.
This should lead to a large amount of unused hash power, that could be made available to some majority attacker.
What are the implications of just knowing that someone (although unlikely) could be covertly using this hash power to mine a parallel and longer blockchain before difficulty adjusts, causing all sorts of trouble?
Can this scenario be remedied somehow by altering the protocol?