Here is a quick decklist I have come up with after I browsed through the possible sleeper cards in Rivals of Ixalan. The card that I have considered is Hadana’s Climb//Winged Temple of Orazca. With Winding Constrictor to easily trigger the flip of this card, having the temple early not only fixes your mana but also can get your biggest creature to evade past chump blockers or even the big and fatty Dinosaurs. Check out the decklist below.

Maindeck
Creatures
4 Glint-Sleeve Siphoner
4 Merfolk Branchwalker
3 Rishkar, Peema Renegade
4 Servant of the Conduit
4 Verdurous Gearhulk
4 Walking Ballista
4 Winding Constrictor

Spells
3 Fatal Push
2 Vraska’s Contempt

2 Aethersphere Harvester
2 Hadana’s Climb

Lands
4 Aether Hub
4 Blooming Marsh
4 Botanical Sanctum
2 Drowned Catacomb
2 Fetid Pools
5 Forest
3 Swamp