Idk about you guys but when I play unholy after i death pact i spend 2 globals after doing death coils into the pet to heal it back full or else i risk it getting killed. So there is a hidden rp cost and globals lost :P.
Conversion - Modified to be an on use ability that consumes runic power to generate a heal over time that heals x% hp per 3 seconds. The length of your conversion is y seconds per 10 runic power consumed. Perhaps a cap of 50 runic power, but a 100 cap could be beneficial in both pve and pvp. You sacrifice damage for a lengthier means of healing, if combined with particular runic power generating talents (ams, horn of winter and empowered runic weapon), it could be quite useful
Or
Conversion - Revamped and now works entirely differently, but retains the existing name since I'm too lazy to think of a new one. Your diseases passively heal you for x% of the damage they deal. With blood dks able to refresh diseases on demand, frost dks having frost fever spread out and unholy dks being dependant on diseases, all three classes can benefit from this. In massive aoe situations, it could become overpowered. Because of this, we can add a cap of 5 targets causing a heal and the actual heal itself can't be too much. It's essentially a self hot so the numbers can't be too large or it risks becoming overpowered which just replaces the existing problem.
I prefer the disease idea personally, with death siphon being a ranged damaging attack that scales with gear and death pact being an on demand heal, this evens outs. We have a big self heal on a cooldown, we have burst heals that deal damage and can be used frequently, and then we get passive heals that will accumulate over the course of a fight. All of them are beneficial in pve and pvp for all three specs.
Not sure how thats a problem lol, I've played strictly unholy in arena and got 2.2 in 3's with it and only played a few games as frost after i got my T2. Seem to always do better as unholy, but I've try to play as frost a few times since everyone says its the better spec.
I think instead of using runic power for conversion they could try something like AotD and use 1 of each rune to activate conversion for a set amount of time. i'd rather waste runes than runic power since ibf and lich heals are so dependent on it.