A while back I played with several ideas related to a theoretical new card type, Terrain. I was mulling the idea over again, and trying to resurrect the simplest form of it.

This version is just a normal permanent that could be an enchantment, except for the rules baggage more clearly conveyed along with a specific card type. It eschews a variety of other baggage I describe below.

The big downside is things that make it harder to attack make the game unfun if they're too strong or numerous.

One thing I left out is the idea of playing terrains on the opponent, which (a) give some beneficial ability like an equipment, but only when you attack (b) let you bypass their terrains. Although to make that work the terrains would need to attach to a player, not refer to "you". (Originally I envisaged a more complicated arrangement where you attacked outward through your terrains and inward through your opponent's terrains at the same time, but that was confusing.)