Opponent can also play their cards for free. Watch it.
I can change it to 'all cards cost 1 of their element', to stop fractal and sky blitz for a while. Unless you're hoarding an air tower. (Before you call OP, remember that this is not in element with any of those.)
One does not simply leave a card without image if it has an Awesome name!
The default for Darkness actually does a pretty good job