Here's an easy fix to being able to use all of your opponents creatures for nothing. Have the card do this; "For each creature you control, you swap that in place of 2 opponents creatures." This way you have to have creatures out already to use his, but it isn't just 1-1 which would make the card useless in many situations. Now, it does what you want it to, and it also isn't overly powerful. Pharaoh can be used to generate scarabs to put on your opponents field, skeletons to give your opponent weak creatures that are easy to get out, etc.
I think that's the best way to go about a card like this. As I said earlier, trading one card with your opponent (Especially at random) is a bit useless. Another though is Malignant cell. Use Malignant Cell on a creature you have, stall, then play this. Your opponent gets the swarm of Cells, you get their creatures. I think if there is no room on your opponents field, the creatures should die when they need to be swapped back. Not sure if that's OP, shouldn't be something you could do incredibly fast, plus they get space when your original cells are returned to your field.