I could see it being tacked onto Legacy of the Frost Witch at this point though I’d actually prefer that talent returned to reset on 5 MSW just to make it more predictable.
I’d also just be satisfied with Stormbringer or one of its talents (maybe storms wrath) also granting a floating charge similar to the way Shadow Priest’s free Mind Blast procs work with Dark Thought instead of always having two charges. This would smooth out the rotation a bit so that late Stormbringer procs don’t feel wasted.