The rest have to be discarded, but it's a lot of aggression for a cost four card
Youll have to decide if you're resigning yourself to building a mono-white Gandalf the White deck, or adding it to your 5-color humans/legends matters deck
While it might be tempting to try to buy the Fires Friends list and drop the planeswalkers to play Jeskai Fires (which is a top-tier deck in Standard), apart from Fires of Invention, Kenrith, the Returned King, Deafening Clarion, and one copy of Steam Vents, none of the cards from the Challenger Deck actually show up in Jeskai Fires
The trick for other decks with an aggressive enough commander is whether the full mana cost is still worth running
The replacement effect from Suplex's first mode will exile the target creature if it would die this turn for any reason, not just due to lethal damage
But just because a card has a big name in one format doesn't necessarily translate to casual town