Per Saffron Olive's Tweet:
"Wizards is testing a new mulligan rule at Mythic Championship London. If you mulligan you choose to draw back up to seven cards and then when you finish mulliganing you put a card on the bottom of your library for each time you mulligan. No scrying."
If anyone is curious, the odds of failing to finding a 4-of in a 60 card deck with 6 mulligans (as 7 mulligans serves no point) is (1-0.40)^7 = 2.8%. This doesn't include Serum Powder and yes, I am clearly referring to dredge here.
This is a huge change and more powerful than the proposed change of "take a scry for each mulligan" as looting improves the quality of cards in your hand and you would have to choose to keep a hand before scrying under the old system. I think this would have a pretty significant impact on mulligan decisions and vintage gameplay and it might just be wrong to keep a hand without Power in Vintage if the new rule is implemented.
And per Hipsters of the Coast's tweet:
Ian Duke says that if the new mulligan rule sticks, Wizards will take a look at the Banned & Restricted list and reevaluate cards that might be a problem/take too much advantage with the new mulligan
So that might stir the pot as well...
Edit: Fixed the math