First of all, let me point out that at this point, this is little more than an idea. I am currently busy programming my League of Legends community site so I do not have the time to start programming a game yet, but I would still like to hear what people think of the idea.
The technical bit
The game bit
I will not be spending any time here discussing a 'story', as sandbox games are not really defined by a storyline you follow. Instead I will discuss the general setting and the game concepts. After all, the story in a sandbox game is what you make of it. Explore the entire world? Capture all the creatures? Become a master crafter? Build a fortress with your friends? It's really up to you.
Not a pokémon game
Odd as this might sound on a pokécommunity, I do not intend to make it a Pokémon game. Instead I plan to make it Pokémon-esque. The reason for this is that while Pokémon games are amazing in their own right, I feel they have too much of a stigma attached to them. People will expect to be able to get badges, will complain when Pikachu is not involved, etc. Instead the game will be a monster capture game in an unique setting.
Big open world
Rather than routes you have to follow, the game would feature a big open world, that loops around the edges. (If you go far enough east you'll end up west, go north and you'll eventually end up south, etc.) If you need an idea of how this works, think a game like Skyrim or Minecraft, rather than a game like Neverwinter Nights or the Witcher. One lets you decide where you want to go and what you want to do next, where as the other holds you by the hand and tells you what part of the story comes next, often boxing you in with walls, cliffs or invisible walls.
If you want to run off and explore to see if you can find a desert or feel like getting yourself lost, that's your call. You might not survive it, but hey, that's part of the adventuring life, isn't it?
Gather wood, mine for ore and rocks, gather flowers and put them to good use! Craft held items that can boost the statistics of your monsters, build furniture for your personal home or simply try to control the market by amassing a fortune worth of gold ingots.
I do not plan to have your typical 'right click and wait' crafting system, instead each type of gathering would involve a minigame of its own. The better you perform, the better the results you get. The same holds true for crafting. Simply getting 10 iron and turning them into a sword is not going to cut it if you want to make the best 10-iron-worth sword there is.
This ties in with the crafting system to a large extend. It is no secret to anybody that the mass harvesting of crops, the mining of ore and the chopping down of wood is anything but a good impact on nature. The game would reflect this, by tying a value to every area. If you chop down a tree it will grow back over time, but if you continue to focus your attention on a single area, the ecology of that area will slowly collapse. The wood will of lesser quality, fewer trees will regrow and even the monsters you encounter shift.
Players can assist in recovering nature, or they can simply not care and go around their business, slowly ruining nature one place at a time. Just remember that no decision is ever without consequences, not even in a sandbox.
Note: Not intended as visual representation of graphics engine, just a quick example picture I threw together in photoshop.
Why not a normal Pokémon game?
Look around you on these forums, I do believe there are plenty of those being made or already finished. I don't think it really serves any good to divide the community in making yet another generic pokémon game, when something original could be done.