Big Mom and WB both have it tho
if oda gives it to him it will probably just be supplementary and not alter his fighting style at all. Think of shockwille enhanced attacks, amputate that can bypass haki defenses etc
And bigmom and Whitebeard do not Utilise Haki to amplify their attack.
They may use Haki as a " Medium " to touch Logia types but that's all.
Shockwille enhancement will be Law base getting more and more stronger , The way Law powers work , I don't how Law uses DF and Haki simultaneously.
It's like the case with Bigmom and whitebeard all together again.
You can't Soul and shockwaves in Haki , nor gravity.
If gets it , it would be more of a arsenal increase instead of AP / DC increase.