Leaders are always a safe bet to purchase, as they retain their value and sometimes even go up in price depending on the leaders tournament results
There are some corner cases, however
Natural Order does make you sacrifice a green Creature in order to cast it, but as long as youve built up a wide board with mana dorks, casting it is trivial
Yawgmoth's Hospital wouldn't want it since it can't find Yawgmoth, Thran Physician, decks like Elementals have too many colors, and various Arbor Elf ramp and midrange decks are mostly built around non-green creatures
Among the creatures revealed to be in the set are common enemies such as flumphs, major antagonists from D&D history - including five-headed dragon Tiamat and spider queen Lolth - and references to staple Dungeons & Dragons NPCs, such as the Preposterous Innkeeper