I am trying to design a game with some interesting features but one question I have is regarding terrain. I want a terrain that will make for a very large game world, and I want to be able to have such features as "roads" and "rivers".
What options do I have in terms of terrain and what will give me the best result in terms of best looking, obviously I want to also keep the game playable with a reasonable frame-rate, but this game will specifically target higher end hardware.
You should stream your terrain from disk; don't load the entire world into memory just to render what's around the player.
Also see "What’s the newest trend / method for terrain rendering?" and "What is 'ROAM' related to terrain rendering?".
Traditionally, a heightmap grid of tiles has been used to represent the terrain geometry. However it doesn't allow for multiple floors or cave-like structures, though you can fudge it by adding meshes that seamlessly integrate with the heightmap.
As you seem to be new to this, you might want to start by playing with a world editor. There are plenty of Google results for terrain editor that you can try; I personally can't recommend any one of them from the rest but perhaps someone can comment on them.
The existing answers give a lot of information.
I also gave a talk a couple of years back on how the terrain system in PhyreEngine works. It's PS3 oriented, but actually fairly similar to the Halo Wars technique in many ways - we also have full 3D displacement, and indeed can handle "bridges" (i.e. complex topologies). This is available here:
