There is a problem with Thief. There is very little direct competition between players. Has been this way forever. RIght now if you want to take down the player with the most loot to prevent them from winning, your goal is to get to the loot first so they can't take it. But that's your goal for the whole game anyway; there's no change in strategy according to the situation. Not good, needs to be fixed.
I'm going to instate a new type of card to rectify this. The Effect Card. It's an idea that has been going around in different forms for a while, and it has the potential to come into existence now using Option cards but it hasn't materialized so I need to get the ball rolling by giving it a name. The idea is that you play a card that directly impedes your opponent's progress. It lasts for one turn only, like Options. It is played as your opponent comes up with a plan to defeat an Obstacle, and will interfere with that plan, perhaps negating it entirely. For example, Jim is facing a Giant Moat Filled With Barbed Wire and is using his Winged Kicks to fly over it. All of a sudden, there's a hurricane! Frank has played an Effect card that changes the weather to something that prevents the buoyant sneakers from being used. Back to the drawing board for Jim. It stays raining for the rest of Jim's turn, which might affect his plans down the road.
There is a nice balancing benefit of the Effect Cards. Create a Cards have until now been used as a get-past-an-obstacle-for-free card. You have a problem, a CaC will fix it. Now, there is a strong incentive to hold on a CaC to prevent other players from advancing, particularly if you are not in the lead. Players will have to way the pros and cons of using a CaC on their turn or holding on to it.