How about for Crystal Ball, it's not you making the choice. Instead, it's the player to the left deciding. It gives it a (necessary?) nerf for when it turns up a non Crystal Ball card, and for the crystal ball he has to choose whether you get the treasure bonus now or next turn. It's also a unique effect not yet seen on an official dominion card.
Also on another, contradictory note, I do love Harbinger, but it does strike me as potentially annoying in multiplayer games if only one player decides to go with the strategy. While one player is playing Harbinger after Harbinger, and another player is spending his time deciding on which of the 16 ways to split up the cards would most benefit him (which may even involve kingmaking and metagaming decisions), everyone else has to sit back and watch. Envoy may have this problem, but it's terminal and only requires picking one card, not splitting up 5. I know there's no precedent, but maybe for both this, and my suggestion for crystal ball, you can nominate a player to do the choosing. That could lead to some political problems though.
Edit: I don't think crystal ball is overpowered though. If tunnel was a fan card I'm sure we'd all be just as worried.