It is my nature to do what everyone else says I absolutely shouldn't regarding Magic cards, so I'm going to play Dubious Challenge in a 60-card deck.
I figure the best way to take advantage of it is to fish for big, smashy creatures without severely one sided ETB effects since I'm going to be giving my opponent the chance to borrow them. Melding Bruna, the Fading Light and Gisela, the Broken Blade seems like a cool thing to do and that also gives me the possibility to pull off Angel-shaped graveyard shenanigans.
I may want more ways to load my graveyard other than Vessel of Nascency, since I can resurrect Angels with Bruna or any nonland permanent with Emeria Shepherd, but I don't really think that's Plan A.
The key to getting my creatures back is Eldrazi Displacer, thus the four-of plus From Beyond to search it up. In a pinch, Long Road Home can also get me creatures back under my control.
I figure Subjugator Angel is probably the best choice of target for my Displacers when not dubiously challenging so I can basically keep my opponent's creatures tapped down permanently as long as they're both online.
Cataclysmic Gearhulk still wipes out my opponent's side of the battlefield even if they take it from Dubious Challenge (although they can end up with a Cataclysmic Gearhulk out of the deal), so it can possibly turn my Challenges into almost-sweepers.
I don't really play Standard, so I'm classless on what I may even want to sideboard against.
