Here's some ideas. Will edit this post as more are thought of.
Air monolith: Attracts airborne creatures (like a bird resting in a nest.)
Perhaps creatures in the air 'rest' on the monolith for a while? It's simple enough to flesh out the Attract or Nest idea.
or
? - In the sense that it's either a very strong monolith/obelisk, earth would make sense for a strong material. Aether because of 'slightly immaterial'?
Earth Obelisk: This permanent turns into Ruins when destroyed.
Reconstruct: Rebuild Earth Obelisk (one time use?)
Aether Obelisk: Triality(in the sense of Duality)
: Put a card of an element not on the field onto the field/into your hand
Maybe treat the
one like a giant prison in the shape of a monolith or obelisk, and one creature at a time would be 'captured' (somehow) and... stay there until it dies. in which case the 'capture' ability could be used again.
Edit1: Fire ML or OB (monolith or obelisk) - Treat the ML or OB as a ?holy? place for when creatures die. The creature's body is taken there to be cremated. (this is somewhat a nerf to the scavenger ability) But to avoid redundancy i don't want this to have the same effect as immolation. When a person is cremated, a pile of bones is left, which are often ground into ashes. So the effect of the fire ML/OB is :Whenever a creature dies, it is cremated. Cremate
: produce a pile of bones. The bones could just be 0/1 creatures, or serve a different purpose. (hopefully this isn't going to become complicated)
Edit2: Changed Earth and Aether abilities.