Slimefoot, Thallid Transplant does bring in a spellbook of many great fungi and saproling-related cards but does not meet criteria on its own
But it gets even better with its [V] ability, Ultimate End, which gives all your other creatures +2/+2 until end of turn and puts an indestructible counter on each of them
According to an article by MTG head designer Mark Rosewater, there are a number of reasons for this change
In order to thrive in todays Standard metagame, the deck might be able to get more aggressive with cards like Liliana, Heretical Healer and Despoiler of Souls fuelling an aggressive curve
It was something much more exciting than the typical Core Set, and felt less like a collection of random designs based on high fantasy all thrown together in one set (though there was still plenty of that)
This category includes spells like instants and sorceries, along with permanents like creatures and enchantments