Hearthpwn

Gonna figure out a way to condense this one more.  No other druid secrets to confound opponent? This one is two!

Allows hunters to reveal up to 4 cards in the opponent's hand, but can be played around to reveal less so as not to be too op

Flavor text: Neptulon doesn't like it when his minions are summoned to do someone else's bidding.

With both Vaporize and this in a deck, the opponent won't know if it's better to attack with the weakest or the strongest minion first.

Priest secret.

Edit: fix speling

Amplify Magic for Mages. Use your opponents spell to empower your own! 
Either fill the board with cheap minions to get max value, or very durable/stealth-minions.

I think it is balanced at 0 mana. Your opponent can play around it by leaving one empty mana crystal, netting you just one Coin. It also has the potential to overfill your hand and deny you a draw. 

http://i.imgur.com/mcuCTR7.png

The card will not activate if you don't actually have a weapon equipped.  it is 4 cost because warrior is already strong.  could be 3 cost though.

I figured I wanted to try for a different kind of secret, one which does something until it is triggered.

EDIT: Changed the end condition to taking damage (like all secrets, not on your turn) - that way you cannot use the effects of this card to protect this card from being disabled. Once you play a "trained" minion your opponent will know what they need to do to end the secret and can probably do it.

Here's my submition. It has obvious synergy with Acolyte of Pain however it's quite bad against low attack minions.

Some extra explanation is required:

1. Yes, when selecting this Secret you get to pick one of your friendly minions to be able to trigger it.

2. No, your opponent should not be able to see which one you pick. The targeting arrow should not be visible, nor should any buff be visible to your opponent on the minion you selected.

3. Yes, you can nullify this secret when Polymorphing/Hexing/Silencing  the minion that had the buff, which reveals the secret.

4. No, the Charge on the Doomguard barely has effect, so it's not too overpowered. Remember that you need board control for this secret to be powerful. You need one minion out for it to trigger to begin with, and you need 2 minions out for your opponent to have to do any guesswork.

5. One thing to keep in mind is that the Secret portion of this spell will often prevent your opponent from killing minions if they do not have an answer to the Doomguard, in fear of the Doomguard spawning. This is what makes it so powerful.

 Edit: Made it 5 mana, or it would be too strong.

Secret: When your opponent kills your last minion, summon Water Elemental

Example: Your board: 2/3 Snowchugger, turn 3 you play this secret. Opponent on your next turn kills it with Darkbomb. Because it was your last minion on board, secret is triggered and Water Elemental is summoned.

This secret actually brings some board control even after your opponent clears your board. Water Elemental originally cost 4, but because secrets have conditions to be met, three mana is in my opinion very acceptable for this secret (just like other mage secrets)

If you dont have any minion on board, when you play secret, it wont trigger, because your last minion was not killed. It also wont trigger if your last minion is for example Mind Controlled, because its not killed!

Some interesting results can be done with deathrattle minions, lets say with Piloted Shredder. If your opponent kills it as your last minion, while secret is active, you get your water elemental and Shredders 2-cost minion.

I hope you like it :)

Edit: Wording.

i cant wait no longer...

damage up to 4

Demonpact

3, Warlock Secret, When a demon dies summon a copy of it.

(Edited a few times for typos and better art.)