Well if I learned anything from the Game Maker community it was this: if you are looking for something like Game Maker or Sandbox, your looking for a starting point, somewhere where the complicated base points are already covered and you just change things to how you want, and only the advanced users who have learned from changing pre-built games are looking to build from scratch.
There are a lot of users that are advanced from point A but they will probably go with something like Unity or Visual Studio, unless they are looking for something that will help speed up the design process.
Of course a scripting language the user can use would be needed (preferably something simple like Python, XML, or C++), but also there should be a menu thing for those of us who are still learning and need that starting point for every object, or even just certain ones that we can just mess with until it works like we want.
This is what I think would be a simple way of going about it from how the end user would be seeing it
a per-object right-click menu, so like you have 3d object A, you right click it and you can edit it's script, or choose what it is, character, monster, vehicle, decoration, weapon, effect object, etc. (let's say we choose vehicle)
Then it goes into the sub-categories like car, space ship, train. (let's say we chose car)
Then whether it's a player, NPC or something in-between the two. (let's say we chose NPC)
And lastly the attributes like top speed, acceleration, weight, a driving line map, crash physics type, crash avoidance type, etc.
Going through all this will erase any script for that one object, if there is any, and put in a new script built off the choices the user made. Which the user can then come back and edit how they please through the 'edit script' option on right click. This idea is also good for a fresh start on an object if you mess it up beyond repair.
And the scripts setup by this process could have notations in it that explain how each part works to newcomers.
Kinda like a cheat sheet sort of deal.
Plus this method makes it easier for your guys/you to add more pre-built scripts on a whim.
(removed something in this area as I realized it was stupid and pointless)
Advanced users would also want an optional side menu you can use to select an object that's already placed and edit it or move to it, to save the hassle of flying around a gigantic map.
This way it makes the main parts of code simple and fast for not only the ones trying to learn or just mess around, but it will be quick and convenient for the advanced users as well.
Since sandbox is a 3d editor there's also the level designer, the world needs it's own scripts and what not, like being able to right click the ground and edit and you get a brush thing and a menu to paint the height and texture of the land, Have a bucket to fill land indentions with water by clicking and/or holding down, and have it so if the water flows with gravity so if it is going downhill it will be like rapids, and if it goes off a cliff it's like a waterfall, and a raise/lower paint brush for the water if you want to create some sort of magic water wall or something.
The simple way is to set the height of the water layer, but if you do that any lakes will have to be lower than any underground cities or mines, caves, etc, and as games like Wurm online show, that does not make for a pretty landscape.
Paintbrushes for quick painting/generating forests of different types with rocks and trees and whatnot, with the option as well to place such things individually.
And of course custom texture and paintbrush shape options (which would easily be done through .png files)
And the parts on objects that you see in the designer that let you drag it, re-size it, rotate it, etc, in whatever direction based off an axis, for simple placement in a 3d world.
Plus a weather/climate layer editor so you can paint what areas rain, snow, cycle, rain fire, etc. And weather scripts as well if you want to make your own weather, like say, raining dogs that are on fire (Elder scrolls 4 reference).
Oh and land layers (just came to mind) where you can create a second, third, etc, ground layer at a specified height. Like if you want to make floating islands, so you won't have to do it minecraft style and have it held up by this one random tiny pole that you expanded the top of to make the landmass.
And an option for no land layers so it ends up nothing-ness like space so people can put planet 3d objects for space games and such.
Again though just my ideas from my experiences and people I have talked to, and I realize some of these ideas are already implemented, I just mean as a whole on how it would work in the idea.
And I'd be glad to join in on the meeting if you would be interested on my thoughts in the matters, including/beyond what has been said in this thread.
Just as long as it's not an in-person thing, I don't exactly have the cash to be traveling wherever on a whim
But Skype, Yahoo, AIM, forum thread, IRC, whatever is cool with me, if I don't have an account with the place you prefer I can make one. (I may need a short tutorial for IRC if you go that rout and I have to use a download client, I tend to get confused with that thing
I find it odd that we're the only ones posting here...
HEY, OTHER PEOPLE, I know your reading this, don't let me be the only end-user showing interest in improving the program, post some ideas, thoughts, if you have a better idea then me say it.
Also I was thinking about the scripting language for the end-user to use, it will probably be the most important thing to change because it will effect pretty much everything, but if you do something like XML, you're basically gonna have to make your own scripting language based off it for any decent 3d support, like Game Maker did (they used GML which is a modified XML). I mean it's an option if you really wanna put your foot in it, but definitely not a easy thing to do.
So perhaps a better option would be Python or C++.
The problem between the two is that while Python is easier and faster to write, C++ is going to have more capabilities and better performance.
If it's going the cheat sheet rout like I mentioned earlier in this post, it may be best to do C++ for the extra features and performance, and users can continue to use the menus until they have a good enough grasp of the language to personally write it.
One more thing that just now came to my mind, I don't know what it's called but you know how in RPGs there are different areas? It's not just one oversized open world, even if it seems so? That ability, so the areas can be made smaller for weaker computers or mobile devices, and it makes it easier to do things like other dimensions for instance in elder scrolls 4 how the shivering isles is a completely different map then cyrodiil, for many reasons.