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Overgrow
Instant
Target opponent chooses one, and then you activate it - Put 1 4/4 Green Beast token on the battlefield; or target opponent searches your deck for 2 lands and puts them on the battlefield under your control tapped.
"You feed nature, or nature feeds on you."

Interrogate
Instant
Target opponent chooses one, and then you activate it - Counter target spell; or target player draws 2 cards.
"Tell me what you know, or I will make sure you don't know anything."

Victimize
Instant
Target opponent chooses one, and then you activate it - Look at target player's hand, and discard 1 nonland card; or target player discards 2 cards. 

Reinforce
Instant
Target opponent chooses one, and then you activate it - Put two 1/1 white Spirit Tokens with flying onto the battlefield; or put one 2/2 white Gryphon token with flying onto the battlefield

Recklessness
Instant
Target opponent chooses one, and then you activate it - Cast the top card of your deck without paying its mana cost; or exile the top two cards of your deck, and cast any instants exiled in this way without paying their mana cost.

All but the Green and Blue Ones are a little underpowered.

Black should be either a Zombify or a Mind Rot.

What you mean with "and then you activate it"?

Okay I'm going to fix these up a bit.

It seems my wording got me into trouble, and I screwed up looking back at it. I was trying to go for "you can either give me 1 good thing; or you can give me 2 not-quite-as-good things." Which is a bit awkward, though I like the black one. Instead, I am going to try for "you can either give me what I want (give me a good thing), or i'll hurt you." Note: putting out a creature apparently = hurting you. Except for some reason in the case of black. Or maybe black just feels like hurting you either way. IDK.

Anyway, here's the new versions:

Overgrow
Instant
Put 1 4/4 Wurm token on the battlefield unless an opponent chooses to have you search your deck for one land card and put it on the battlefield tapped instead.
"You will feed nature, or nature will feed on you."

Interrogate
Instant
Counter target spell unless an opponent chooses to have you draw 2 cards instead.
"Tell me what you know, or I'll make sure you don't know it."

Terrorize
Instant
You look at your opponents hand and exile 1 card in it unless an opponent chooses for you to put one creature from any graveyard onto the battlefield instead.

Reinforce
Instant
You put 2 1/1 white Spirit tokens with flying onto the battlefield unless an opponent chooses for your life to become 20 instead.

Recklessness
Instant
You deal 5 damage to target creature or player unless an opponent chooses for you to play the top card of your library without paying its mana cost instead.

I especially like the red one, I think it'll make for some nice multiplayer politics. They all will, potentially. You can get one person to help you, and get off many useful effects.