I think there could be a transparent darkness shadow/smoke (think of flooding visual effect) to the opponent that cover's the creature field and permanent field while the cloak effect is under effect. This would be cool and look like darkness is hiding all the creatures and at the same time avoids the strange feeling of seeing empty field.
It is quite disturbing to see all of the opponent's creatures and permanents just disappear suddenly without any effect. It looks like a glitch or crash (to beginners) to see an empty clear opponent's field.
