If I had to predict what will happen, I think Serum Powder might get restricted preemptively. I actually think Dredge will be fine from a balance perspective within the Vintage metagame...the failure rate with 1 Powder is going to be slightly lower than it was before with 4 Powders and the old mulligan rule. This gets countered by increased odds of finding hate. Dredge is probably a little better but not oppressively so.
I think in terms of other decks, the average quality of a mulligan goes up considerably and you'll see more broken openers and consistent combo draws. It will affect vintage play significantly.