Despite Proliferate being a major mechanic in Phyrexia: All Will Be One, the set doesn’t have a single +1/+1 or -1/-1 counter in it. Instead the main counters at play, working with proliferate, will be poison, oil counters, and planeswalker loyalties.
“There are neither type of +1/+1 or -1/-1 counters in the set,” designer Adam Prosak tells Wargamer, explaining that oil counters largely take their place, as these can be used “for a wider variety of benefits in conjunction with proliferate.”
As Mark Rosewater explains in his Phyrexia: All Will Be One direction piece, oil counters are “quite flexible”, with all sorts of different effects, from changing the size of a creature or power of an effect, to allowing threshold abilities to go online, to helping you count down. “They are a resource and different decks will be able to manipulate them in ways that lead to compelling gameplay.”
This seems like it’s going to lead to quite the unusual MTG Draft format. My brain’s hardwired to look at proliferate and think about how it’ll affect my or my opponent’s creatures if they’ve got plus one or minus one counters on them. Instead, you’ll have to weigh up a multitude of different card effects based on what oil counter cards you’ve managed to collect so far – which could mean quite a bit of mental load.
Of course, outside of Limited, the influx of proliferate cards will be free to go wild with the full range of counters in Standard right now, from +1/+1 to lore counters on Sagas. It seems quite likely, in fact, that recent sets introduced both shield and stun counters because of the way these could play with proliferate, so expect some proliferating MTG Standard decks heading your way soon.