Here's what I was thinking:
At character creation Heroes have a choice to take an extra 2 points of hindrances and gain the use soulstone ability (Kindof like Arcane Background)
Character starts with a soulstone with 10 pp (or more if you want) then selects one power from the Necessary Evil book. Every time the hero uses that power it uses its base cost in power points from the soulstone (power modifiers will increase or decrease the cost like normal). 1 point may be spent off the soulstone to make a soak role (or if you wanted to be more Wyrd, flip a card and heal.) 1 point may be spent from a soulstone to add a d6 to a roll.
When the soulstones points are expended the stone cannot be used again until it is recharged or the player finds a new one. To recharge a stone the player has to be withing 6" of a dying creature, The stone then absorbs the soul recharging an amount of power points to the soulstone equal to either its rank, difficulty, or spirit dice (d4=1,d6=2,d8=3,d10=4,d12=5). GMs call.
At each new rank a player may take Extra Power (or name it Soulstone Mastery or something to fit the setting) which will let the player select a new power to use with a soulstone. This is to represent a character learning how to harness and use the soulstone more efficiently and is able to draw more innate powers out of himself. Extra power points can not be taken for soulstone abilities. This way players want soulstones more than anything else to increase their own power. It will also make Players and Npcs alittle more cutthroat and dangerous which I think reflects Malifaux better.
Players that dont take Use Soulstone at character creation may still use soulstones but do not get to select a power or start with one. These characters may use 2 points to make a healing roll or use 2 points to add a d6 to any roll (you can make the cost one if you want but it might make things alittle unbalanced). A character can take Use Soulstone whenever he or she gains a new rank then is allowed to select one power like normal.
Hopefully all of that makes sense