Lego themed building RPG

[Myrddin]

Hi Fellow "Sandboxer's"

I have been looking for a long time for a decent "lego" themed 3d environment based game / simulator, yet even though I use several "Lego" like cad packages, there is that interactive element missing where one could explore his or her creations as though she / he was a lego figure. So therefore I've decided to look at developing a Lego themed Building Game which would be along the lines of being an RPG where ideally I'd like to produce possibility for the player to be able to do things within the "3d world"

the idea is to start with a city themed building game where you'd be presented with a 3d environment where the player would be able to not only construct anything of he or she desires, now Multiplayer is also something I'd like to incorperate where players would be able to build coperatively and also take on various roles within the 3d world, such as
a teacher, chef, policeman etc

actual game play ideas are still being thought up within regards to the theme of this type of game, yet what I'd like to know from more seasoned users if the following would be possible?

1. a customised GUI (Graphical User Interface) container, that contains images of the bricks, plates, and other parts which clicked would appear in the 3d world and then moved in place with the use of a Grid system, is it possible to have some sort of a "Snap" system where these "objects" would connect like lego bricks? or would this require extra scripting or programming to be able to so?
2. Additional to the gui a color wheel where brick colors would be changed.
3. interactivity with objects (lego parts) like hinges, doors, windows etc
4. some ability to save what's been made and either load offline or continue to build after saving.
5. the ability to ride / use creations made with the objects like Cars, get on the train etc

Planned features
Working Train / Monorail system
Cars that can be driven including on the "roadplates"
doors and windows that open
Flying aircraft that can be flown
lights (street lights etc)
Day / night effects including weather
AI characters that can be placed (eg. chef, doctor, policeman etc)

Now the part / piece objects for me is easy, as they are all in *.obj format, what I want to know is it possible to add an object within game linked by image on a customised GuI? as well as change colors from the gui too?
would there be a way to snap the parts, either possibly by having some kind of setup on the parts that each part connects to the various "nodes" on each part
the ability to save what's been done

I look forward to ideas and advice toward this as this would be a great bit of software for those whom not only love lego yet also something unique as there isnt anything close to what I'm looking to develop..

Regards
Gareth