Like most Magic: The Gathering cards, the value of Plains depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (The Lost Caverns of Ixalan)
Testing Nicol Bolas, Dragon-God MTG proxy can help you decide whether your Grixis deck can cast and defend him consistently
A lot of these cards would change the format for the better and allow for new avenues of gameplay and deck construction
Hydroid Krasis Once a beast in Standard (literally), Hydroid Krasis is still respectable in any format where dumping a bunch of mana into a serious threat that draws you a couple of cards is a worthwhile investment
Inalla, Archmage Ritualist Inalla, Archmage Ritualist is in my opinion the most poorly designed of the eminence cycle because you don't need to cast her to have access to the vast majority of her power
That is the idea behind the creature card Risen Reef, which perfectly depicts such an impossible phenomenon