Mesa Monument

Some deserts used to be Oceans, so having remnants of them would be reasonable in a Mesa Biome. Mesa Cliffs are large enough that a monument could be carved in there. This Mesa Monument would be built out of red sandstone & the hot variant of prismarine: Sunstone (or Setrine) 

The Mesa Monument would have pottery sherds, trial spawners & vaults, and even a new trim- Sun Trim. Instead of guardians, Helions (or Sentinels) would be guarding the area.

https://feedback.minecraft.net/hc/en-us/community/posts/24489393263373-Helion-Guardian-Variant-in-Desert

After beating the monument, one might be rewarded with the Heart of the Sun: When used in the same conduit configuration, it would grant the player immunity to burning, increased lava swimming speed, strength 1, and burn all hostile mobs near it. All sunstone blocks in the area turn to light blocks.