Cost runic power (frost, not unholy), Proc killing machine during obliteration just like froststrike and howling blast do, And make it instant, no 5s delay on frost, just make it our alternative runic spender in execute range, Froststrike should be about farming pillar and doing damage, while soulreaper should act almost as a superior froststrike at lower healthpools
alternatively if it doesnt become instant proc, and blizzard wishes to keep its “identity” then just lower the time to maybe 2.5s or 3s
or make it instantly proc at another threshhold lower than 35%
So like soulreaper works as normal, 5s and then boom, but if at any point they reach 10% while soulreaper is active, its instantly consumed boom
But for frost to use it, it gotta proc obliteration
For unholy, i mean i think its fine as is, i even get way more procs in pvp as unholy than frost with it because how unholy does its damage, But if blizzard wanted to rework it anyway for funsies they could make it more of a dot for unholy, add into morbidity and function with plaguebringer
Edit: MAKE DND CLEAVE SOULREAPER TOO