3/29/2017 - Here's a keyword for suicide black and dead guy red. These colors care more about hurting their enemy than themselves, so they're happy to pay a few life to make their creatures more aggressive.
While testing might show 2 life sufficient or 4 life necessary, I feel like 3 is a pretty good guess given the above cards. And I feel like bloodpact is a pretty attractive mechanic for Spike.
Next I wondered what this keyword would look like on non-creatures.
For other permanents, you can simply trade the +1/+1 counter for a charge counter, or its non-union bloodpact equivalent.
Spells proved a bit trickier, but we do have an option:
"Increase numbers in its text box" is some real sketchy rules text, but it can definitely work with the kicker template, so let's focus on the viability of the end result first:
Blood Letting is a pretty solid design, dealing 2+2 or 3+3-3. (Maybe it needs to cost 3; that's Dev's concern.)
Instead of paying 2 life like Night's Whisper, you pay twice as much mana for Twisted Advice. Or you can draw 3 cards for 3 life like Ambition's Cost. That first mode isn't a slam dunk, but it works, and the second mode helps carry the blood-for-knowledge flavor black so loves. I'd argue this card is well-balanced.
Blood Scent is worse than Epic Confrontation at common but comparable to Savage Punch and clearly better than Wild Instincts. It's a bit of a push to try bloodpact outside black-red—justifiable—but a push nonetheless. Luckily, red has access to fight too, so a push isn't required if the set doesn't want it.
I'm personally sold on the creature version of bloodpact. The non-creature permanent version seems fine too. The sorcery/instant version has potential, but I wouldn't fight for it and could certainly be swayed against. What do you think? Design a card to support your opinion.