The introduction to The Brothers’ War into Pioneer will change a lot about the format. Many established decks receive clear upgrades with the new set, and plenty of other fringe or entirely nonexistent strategies have the potential to become real contenders with the influx of such a powerful set.
But perhaps the thing that could impact Pioneer the most, at least in the short term, is a land first printed in Ice Age. One of two such lands that complete a cycle of the worst playable dual lands in the format: the pain lands. I’m speaking, of course, of Brushland.
Brushland will not just improve certain existing Selesnya-based strategies, at least until it’s likely replaced by Razorverge Thicket in a few months; the increased mana consistency to the color pair may be enough that brand new archetypes could emerge to compete with the rest of the meta.
Underground River, on the other hand, won’t make nearly as much of a splash.