So i was reading this thread i a saw allot of wonderful ideas. Which is quite logic, just the idea of having the whole world at your disposal makes people dream. But the truth is that a big world doesn't make a game good. I realized that after seeing this video on youtube.
Does a big open world make a good race game? I don't know, but there are some people that knew how to do it right. From other games with big world we learn what people like to do in open worlds. Sim, survival, exploring, creativity, Role-playing, Multiplayer... are just a few keyword here.
SIM/Role-playing
I've never actually played any simulation games but this game just begs to be at least a flight-sim, but even better: a Omni-sim. Again i have no idea what people want from sim games but i think i could enjoy a flight-sim or a whatever sim. But would i want pay $30 for that? I'm not sure, some people would. But not everyone is into sim.
An other thing is a Role-Playing, SecondLife-esque game. People going in the world buying a piece of the land, place you house on it, drive around, chat with people,... basically a life sim. I'm sure there is a market for it, allot of people play The Sims, Second life and RP servers are very popular on GMod. But then again only the people interested in sim would buy it. And i think it would get boring pretty quickly.
Survival/Exploring
I hope this doesn't turn into a minecraft HD debate. But people just like to explore this new world, discover things. But there should be more than just terrain then. Some abandoned temples or ghost towns, Caves en crevasses, these sort of stuffs. Add in some survival element, whether that is survival against zombies or a some harsh post-apocalyptic environment, fun will be guaranteed.
Creativity
like i said before, a big open world is a fountain for dreams and ideas. Let people build and create without the limitations of having to search for resources like in a survival game. What else can i say about a creative game? Not much, go ahead a blow my mind! I'm sure everyone could spend hours being creative.
Multiplayer
i haven't said much about this in the previous ideas. Because this is probable the most important of them all. Everyone should be able to create a server and play sims together, do some co-up survival and share your creativity with other people or create with other people. Multiplayer is a way to keep your game alive. And the best created games are still the multiplayer games, they beat single player hands down.
I really like the concept of GMod, not talking about the gameplay with the physics engine and stuff, but the concept everyone being able to create his own game mod and make a server with it. Everyone just plays whatever they like. This means the game needs a decent Mod and addon support system. If someone wants a race game he just scripts an addon for it, create some tracks with road tool and fires up a server. And if you ever get bored of racing you can always go and play on a different server.
And for the technical side, people don't have to stream the complete map every time you play the game, just like you do in minecraft. The map itself is still the earth and always the same, so if people would download the map when they install the game the server only have to handle the vector data and maybe some models. The filesize of the earth map is only 12GB in total, add in some models, textures and vector data from things like lakes and caves. And ofcourse the code itself. you might get a total of 14~15~GB, which isn't allot these days. Compare that to world of warcraft who is 26GB+ in size.
Anteworld
Maybe you shouldn't sell it as a game, but as a sandbox. Like GMod, but with different gameplay. Start with what you have now, a flight sim, building roads and towns, drive around and jump of cliffs and making awesome wallpapers. The goal for the release should be better lighting, weather systems, biomes, some basic physic engine, ect... Make it look beautiful and let other people create the mods.
You should hand out the SDK and tools before you release the game to some people who are willing to make mods so that are some bound to be ready when you release. Or make some yourself.
Anyway, this is my 2 cents. You don't have to listen to any of this and do whatever the heck you want to do with your engine. It's up to you.