I think Contrary already mentioned something like this, but if we want to add more denial into the game, I think it should not be direct denial.
By direct denial, I mean things like Discord or EQ that literally prevent you from playing anything. It's just no fun when you have your middle finger plotted on the spacebar to end turns because you can't play any cards at all.
Instead, denial I prefer should not prevent you from playing those cards, but cause a negative side effect should those cards be played. This is more mentally challenging, more fun and thus better than adding in direct denial.