Quicksilver Elemental

What it steals is the rarest currency in the game: other creatures' activated abilities, repeatable for a single blue every time. Where most ability-borrowing effects copy a single trigger or grant a keyword, this Elemental absorbs the full activated-ability suite of any creature it targets, friend or foe, and the second clause quietly removes the friction that would normally make that impractical. Blue mana pays for any colored activation cost, so the tap-and-pay abilities of green mana dorks, white pumpers, or red firebreathers all run off your Islands without a single off-color source. The conceit is genuinely strange for blue: it turns the color's deepest weakness, the lack of homegrown combat and toolbox effects, into a parasitic strength that scales with whatever else is on the board. There is a hard ceiling, though. It copies abilities, not power or toughness, so it cannot inherit a creature's body or static keywords; it is a vessel for verbs, not stats. Its value never rises above the activated abilities sitting across the table, and against a board of vanilla creatures it stands there as a 3/4 with nothing to mimic. That narrowness kept it from ever warping a format, but the card stands as a clean expression of a recurring blue ambition: be every other color at once, on your own terms, paid for in your own mana.