@ajfirecracker Do you think Dredge would still run Serum Powder if you have to put the cards back prior to mulliganing?
Hard to say.
I think Powder is actually better with this rule but so are normal mulligans, so it's hard to say how that all cashes out. Note that the cards are put back on the bottom, so Powder is fully effective to dig, and you find Powder more often on more cards, and you can more often avoid exiling multiple Powder for a single effect. So we have 1 neutral factor, 2 positive factors for Powder, and 1 "negative" factor in that the effect is not as sorely needed.
More Powders does also mean more cards in hand on average when you find Bazaar, which is worth a lot when you have disruption elements you can cast from hand.