Although all would be useful, probably very hard to actually implement - though obviously do-able. A construction kit is simpler, because buildings are generally made up of similar components, windows etc.
If you had a gun creator, most parts of gun would only accurately fit the odd model?
Same applies for most vehicles, you can't pick a door thats the wrong shape.
As for a general Entity creator, is that not just any 3D-Software? I'm sure they could create something to easily model any entity, but when there are so many modelling packages around, if it was possible to be any easier it would probably have been already done. They are working on an importer which will do all the required associated files.
Custom Mac Pro (2014): OSX 10.10 (Yosemite) and Windows 8.1 Pro:
Octa-threaded Xeon E5 v2 @ 3.7Ghz [9775]; Dual AMD FirePro D500 (3GB VRAM Each) [3769]; 12GB RAM [2517]; 512GB SSD [9074];