Opposition Agents ability to Stifle fetchland activations and steal tutor targets makes it one of the biggest three-mana threats since True-Name Nemesis
There are actually several versions of Gandalf in the set (including Gandalf the White, whose color identity will not surprise you), and we show him at different points throughout the story by making him different colors
It also lets you have any number of copies in your deck, so you can exploit that if played right
Being a respectable body as a 5/5 Flyer for 6 mana, it almost has a throw-away line of "no maximum hand size"
The potential of a 2-mana dork that taps for 5 mana is obscene
At a Commander table, 3 out of 4 players in a pod playing Golos, Tireless Pilgrim has an immediate, negative impact on the format