I always thought world enchantments were a cool idea, and I still think they could be
For now, Metamorphosis Fanatics price will ebb and flow before skyrocketing in demand
This makes sure that you wont run short on Rat playables
I already made the Prosper, Tome-Bound joke, but adventures gel with anything that cares about casting spells from zones other than your hand
Its cheap, efficient and cant be countered
In fact the printing of Unholy Heat (and to an extent Solitude) has mostly pushed out cards like Tarmogoyf out of the format, as the removal in the format is now so efficient that creatures either need to be one-mana, so you trade at an even level (Dragons Rage Channeler and Ragavan, Nimble Pilferer), too large to be killed by Unholy Heat (Deaths Shadow and Murktide Regent) or generate immediate value upon being played (Solitude, Fury and Omnath, Locus of Creation)