Mean Streets of Gadgetzan presents a new Mage Secret: Potion of Polymorph. It transforms the next opponent minion played into a 1/1 sheep. While the card does cost (1) less than Polymorph, its effect is rather passive and cannot prevent Battlecries. Disabling an enemy Deathrattle or end-of-turn effect sounds promising, but how often will it be worth entirely sacrificing Turn 3 tempo?
Mage can use 7 Secrets in Standard - 6 from Classic plus Effigy from TGT - but it is safe to say only Ice Block is competitively popular, with Ice Barrier in Freeze Mage a distant second. Mirror Entity faded alongside Mech Mage; Duplicate, Fatigue. None of Effigy, Counterspell, Spellbender and Vaporize lasted long after their short times in the sun.
Since Naxxramas was retired to Wild, Mage Secrets have had a hard time; no card could replace Mad Scientist, not even Kirin Tor Mage. This is partly because Mage Secrets are painfully easy to play against. Mirror Entity can be dealt by playing the weakest minion; Counterspell, by playing the least valuable spell (even coin); Vaporize, by going face with the weakest minion; and Effigy, by killing the weakest enemy minion. Unless Mage receives something as effective as Eaglehorn Bow, Cloaked Huntress, or Mysterious Challenger, the pitiable plight of its class Secrets will likely not change.
It may be too early to judge Potion of Polymorph before Mean Streets of Gadgetzan is fully revealed, but as of now, its future seems grim. There are no cards that can synergize with it, nor are there decks it could possibly fit in. It would be disappointing if this card was relegated to appearing only from Babbling Book and Yogg-Saron.