Karn's Bastion

Repeatable proliferate was never new: Contagion Clasp, Contagion Engine, and Thrummingbird had all made the mechanic an activated or triggered engine years earlier. What this land changes is the resource it costs to run one. Those earlier engines occupied a spell slot or a creature slot, a card you had to draw and commit; folding proliferate into the land drop means the engine costs you nothing your deck was not already spending, and it recharges every untap step. That is the reframing that matters: a proliferate you own permanently, accessible from an untapped permanent, stops being an accelerant and becomes the plan itself. It underwrites anything that wins by stacking a resource one increment at a time: loyalty accumulating on planeswalkers, an army of counter-boosted creatures, poison ticking an opponent toward ten. The four-mana activation is what keeps that repetition from being free, and the land taps only for colorless, so it asks a genuine question of a manabase that wants specific colors early. Because proliferate lets you choose which permanents and players receive counters, the effect is selective: you grow the pieces that matter and leave an opponent's charge counters or fading markers alone. The insight is structural rather than novel: not that proliferate could recur, but that hanging the recursion off a land turns a mechanic you had to find into one you simply have.