Thinking back to the games I have played in the past, none have given me more pleasure than the Tribes series. The game revolved around simple principals. It was first person, you had a jetpack, and you could slide along the terrain to gain speed.
Firstly into a little history of the game. Tribes 1, was a game produced by Dynamix. The idea of 'skiiing' along the terrain was not a feature they intended in the game. It was a bug in the engine that, if you tapped jump, you could slide along the terrain. This in combination with the jetpacks, opened up an unparalleled amount of freedom, not yet seen in any game to date. Other games of the era, such as Quake2, were linear indoor games and had next to none of the sense of freedom. Third party scripts were produced allowing the player to always 'ski' along the terrain, gaining momentum.
This game principal, combined with fast paced and skilled fighting in such game modes as, Capture the Flag, and Deathmatch, created the most unique gaming experience I have encountered.
So in short, the level design was huge open terrains, that allowed the player to fly around and get into conflict with the opposing team as they battled for objectives or to capture the flag. I feel this would create an interesting dynamic for terrain. Terrain which is built for an object to move around on freely.