I like to use cards with different but not necessary powerful mechanics, so I would add a new legendary card to Hearthstone called "Mirage Caster", with the Battlecry "The next minion you play cannot be seen by the opponent until your next turn.". This means the next minion is played kind as a Secret, being represented in the board as a "big interrogation point minion" until your next turn, where the real identity of the minion is revealed. That would be very strategic AND fun, since the opponent would always think "What's under that interrogation point? Should I deal with it or just ignore it?". Note that the effects of the minion would work as usual (battlecries, or end turn effects, for example) so it's possible to guess the identity of the minion if his effect it's activate (like Ragnaros, for example). I can see this new card being used in both competitive or just for fun games, allowing a lot of other strategies to be played in the following turns.